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ASSAYING 


IN 


THREE    PARTS 


PART    Ist-GOLD     AND    SILVER    ORES;     PART    2d- 

GOLD  AND  SILVER  BULLiON;  PART 

3d-LEAD,    COPPER,    TIN, 

MERCURY,  ETC. 


By  C.  H.  AARON,  Metallurgist, 

author  of 

'Testing  and  Working  Silver  Ores,"  "Leaching  Gold  and  Silver  Ores. 


PART  I. 


SIXTH    EDITION. 


published  and  sold  by 

THE    MINING   AND    SCIENTIFIC    PRESS, 

OF    SAN    FRANCISCO 

1906. 

I  Copyrighted] 


,  Az 

V.I 


14  (  ■S-'^Lh 


o 


o 

< 


TO 

TKe  Mining  and  Scientific  Press, 
J  SAN   FRANCISCO,   CAL. 

o 

O  THE    STEADFAST    FRIEND   OF   TRUTH   AND 

PROGRESS — AN    HONEST    PAPER  — 


o 

I 


THIS  BOOK  IS  RESPECTFULLY  DEDICATED 
BY  THE  AUTHOR. 


C^-^ 


conten;ts 


Page. 

Preface    7 

Introduction     9 

Implements    16 

Assay  Balance    27 

Materials    ; 38 

The  Assay  Office    44 

Preparation  of  the  Ore 46 

Weighing   the   Charge 50 

Mixing   and   Charging 51 

Assay  of  Litharge 52 

Systems  of  the  Crucible  Assays 54 

Preliminary   Assay    57 

Dressing  the   Crucible  Assays 61 

Examples    of    Dressing 72 

The   Melting  in   Crucibles 75 

Scorification 79 

Cupellation    84 

Weighing   the    Bead 90 

Parting    95 

Calculating  the  Assay 101 

Assay  of  Ore  Containing  Coarse  Metal 103 

Assay  of  Roasted  Ore  for  Solubility 106 

To  Assay  a  Cupel 106 

Assay  by  Amalgamation 107 

Roasted  Silver  Ore  Requires  No  Chemicals 108 

To  Find  the  Value  of  a  Specimen 109 

Tests    for    Ores 115 

J\.  Few   Special   Minerals 120 

Solubility  of  Metals 123 

Substitutes  and  Expedients 125 

A  Simple  Assay  Balance 127 

Assay   Tables    129 


PREFACE   TO   THE   THIRD   EDITION. 


In  a  prefatory  note  to  the  second  edition  of  this  work, 
its  author,  the  late  Charles  H.  Aaron,  says: — 

'In  preparing  this  second  edition  I  have  not  been  deterred  from 
making  alterations,  and  (as  I  hope)  improvements,  by  any  fears  as 
to  the  impression  which  may  be  produced  by  the  tacit  admission 
that  the  first  was  not  all  that  it  might  have  been.  Of  those  who 
have  taken  that  as  their  guide,  I  have  yet  to  hear  of  one  who  has 
been  led  astray;  nevertheless,  I  candidly  think  that  all  such  may 
now  be  gainers  by  laying  that  aside  and  taking  this." 

To  the  edition  thus  introduced,  Mr.  Aaron  applied  the 
same  critical  habit  which  had  discovered  and  corrected 
many  faults  in  the  earlier  work.  Page  by  page  again 
and  again  he  went  over  the  book,  adding  and  retouching 
as  new  observation  and  experience  developed  new  knowl- 
edge, and  as  riper  judgment  enabled  him  to  make  the 
meaning  clearer  and  the  instruction  more  definite  and 
complete.  A  copy  of  the  second  edition,  with  scarcely  a 
page  untouched  by  his  discriminating  pen,  was  among 
Mr.  Aaron's  effects,  and  is  now  in  the  hands  of  the  pub- 
lishers. It  has  been  made  the  basis  of  the  general  revision 
herewith  presented,  every  correction  and  addition  of  the 
author  being  faithfully  reproduced. 

This  third  edition,  therefore,  represents  the  matured 
work  of  the  author,  and  we  may  say  with  candor,  as  he 
said  in  a  former  instance,  that  those  who  have  profitably 
used  the  earlier  edition  may  now  be  gainers  by  laying  it 
aside  and  taking  this. 

The  Publishers. 


PREFACE   TO    THE   FOURTH    EDITION. 


The  standard  character  of  this  work  and  the  steady 
demand  therefor,  justify  the  pubHshers  in  issuing  a  fourth 
edition,  revised  and  further  brought  up  to  date,  with  the 
belief  that  in  the  progressive  science  of  which  it  treats 
the  technical  reader  will  find  sufficient  of  value  to  justify 
its  profitable  use. 

The  Publishers. 

San  Francisco,  Jan.  i,  ipoo. 

PREFACE  TO   THE   FIFTH   EDITION. 


The  advance  in  the  present  state  of  the  art,  and  con- 
sequent present  requirements,  have  made  necessary 
revision  and  rewriting  of  this  work  throughout,  and  the 
fifth  edition  is  presented  in  the  hope  that  it  will  be 
accorded  the  same  measure  of  profitable  approval 
accorded  the  earlier  editions  of  this  standard  work. 

The  Publishers. 

San  Francisco,  March  2^,  ip04. 

PREFACE  TO  THE  SIXTH  EDITION. 


The  few  remaining  copies  of  the  fifth  edition  were  de- 
stroyed in  the  San  Francisco  earthquake  fire.  The  subse- 
quent demand  for  this  practical  work  has  emphasized  its 
usefulness  and  has  justified  a  new  edition.  This  has  been 
thoroughly  revised  by  a  competent  assayer  who  has  incor- 
porated in  it  the  results  of  years  of  teaching.  The  book, 
while  retaining  its  former  simplicity  of  style,  is  techni- 
cally accurate  in  its  description  of  the  most  approved 
methods  of  assaying. 

The  Publishers. 

San  Francisco,  December  i,  ipo6. 


INTRODUCTION. 


All  substances  in  nature  are  either  simple  or  com- 
pound. A  simple  substance  or  body  is  one  from  which 
nothing  different  from  itself  can  be  extracted.  There 
are  about  eighty  simple  substances  known.  They  are 
called  elements. 

The  elements  have  a  tendency  to  combine  one  with 
another,  forming  compounds;  this  tendency  is  called 
affinity.  The  strength  of  the  affinities  of  different  ele- 
ments differs.  The  strength  of  the  affinity  of  any  one 
element  for  any  other  differs  under  different  conditions 
of  temperature,  pressure,  electricity,  etc. 

Elements  are  known  to  be  capable  of  assuming  the 
solid  or  the  gaseous  state;  most  of  them,  also  the  inter- 
mediate condition  of  a  liquid,  the  assuming  of  these  dif- 
ferent conditions  by  the  same  substance  being  dependent 
on  temperature  and  pressure. 

The  elements  may  be  mixed  in  any  proportions,  but 
they  combine  only  in  £xed  proportions.  When  elements 
combine,  they  form  a  substance  which  has  different 
properties  from  those  of  the  components.  Two  or  more 
solids  may  combine  to  form  a  liquid  or.  a  gas ;  two  or 
more  liquids  or  gases  may  form  a  solid,  etc.  A  knowl- 
edge of  the  proportions  in  which  bodies  combine,  and  of 
the  conditions  which  determine  their  combination  and 
dissociation,  forms  the  basis  of  chemistry. 

All  the  metals  are  elements;  their  mixture  by  fusion 
forms  alloys.  Hydrogen,  oxygen,  nitrogen,  chlorine,  bro- 
mine, iodine,  carbon,  sulphur,  selenium,  phosphorus, 
boron,  silicon,  are  elements  which,  with  the  metals,  enter 


10  INTRODUCTION 

into  the  composition  of  the  ores,  fluxes,  etc.,  which  come 
under  the  notice  of  the  assayer. 

The  fact  that  the  elements  combine  only  in  fixed  pro- 
portions is  the  reason  why  a  certain  quantity  of  com- 
bustible matter  produces  only  a  certain  quantity  of  lead 
from  htharge  in  an  assay.  The  element  lead  (a  metal) 
combined  with  the  element  oxygen  (a  gas)  in  the  propor- 
tion of  103.56  parts  by  weight  of  lead  to  8  parts  by 
weight  of  oxygen,  forms  11 1.56  parts  of  litharge.  Lead 
combines  with  oxygen,  under  certain  conditions,  in  two 
other  proportions,  but  the  resulting  compounds  are  not 
litharge,  and  the  proportions  are  definite  and  unchange- 
able in  each,  combining  spontaneously  under  proper  con- 
ditions, and  any  excess  of  either  element  remaining 
separate. 

The  element  carbon  also  combines  with  oxygen,  6  parts 
of  the  former  and  16  of  the  latter,  forming  22  parts  of 
the  gas  commonly  called  carbonic  acid  gas  or  carbon 
dioxide.  Under  the  action  of  heat,  the  affinity  of  carbon 
for  oxygen  is  greater  than  the  affinity  of  lead  for  oxygen ; 
hence  if  litharge  and  carbon  are  heated  together,  the 
carbon  takes  oxygen  from  the  litharge  and  sets  lead 
free.  But  this  action  can  only  take  place  between  definite 
proportions  of  the  substances.  If  twice  11 1.56  grams  of 
litharge,  containing  16  grams  of  oxygen,  be  heated 
together  with  6  grams  of  carbon,  the  carbon  takes  all  the 
oxygen,  forming  22  grams  of  carbonic  acid  gas,  and  the 
whole  of  the  lead  is  set  free;  but  if  more  litharge  had 
been  present,  that  additional  portion  would  have 
remained  unchanged.  From  this  we  see  that  6  grams  of 
carbon  reduce  223.12  grams  litharge,  setting  free  207.12 


INTRODUCTION  II 

grams  of  lead,  or  i  gram  of  carbon  produces  34.52  grams 
of  lead  from  litharge. 

Hydrogen,  sulphur,  iron,  and  many  other  substances 
take  oxygen  from  litharge  under  heat,  in  different  but 
fixed  proportions  for  each.  Charcoal  is  slightly  impure 
carbon,  and  i  gram  liberates  only  about  30  grams  of 
lead  from  a  corresponding  quantity  of  litharge.  Flour 
contains  both  carbon  and  hydrogen,  but  as  it  also  contains 
oxygen  and  nitrogen,  it  is  less  efficient,  weight  for  weight, 
than  charcoal  as  a  reducing  agent ;  i  gram  liberates  about 
15  grams  of  lead;  it  is  preferable  to  charcoal  for  assay- 
ing, because  it  is  cleanly  and  in  fine  powder. 

Niter  consists  of  nitrogen,  oxygen,  and  potassium. 
When  heated,  it  gives  off  a  part  of  its  oxygen,  and  if 
lead  is  present  litharge  is  formed,  the  quantity  of  lead 
so  transformed  depending  on  the  quantity  of  the  nitre, 
in  accordance  with  the  law  of  definite  proportions ; 
if  sulphur  or  carbon  be  also  present,  the  oxygen  combines 
with  that,  because  the  affinity  is  stronger  than  for  lead. 

Compounds  can  combine  with  other  compounds,  form- 
ing new  compounds.  The  alkaline  carbonates  used  in 
assaying  are  compounds  of  the  metal  elements  sodium  or 
potassium  with  oxygen  and  carbon  dioxide.  Quartz 
(silica)  is  a  compound  of  silicon  with  oxygen.  When 
quartz  is  heated  in  contact  with  an  alkali  carbonate,  the 
latter  is  decomposed,  not  into  its  elements,  but  into  car- 
bonic dioxide,  which  escapes  with  effervescence,  and 
sodium  or  potassium  oxide,  which  combines  with  the 
quartz — a  compound  with  a  compound — forming  glass; 
thus  quartz,  which  is  not  fusible  at  any  common  furnace 
heat,  is  converted  into  an  easily  fusible  substance. 

One-fifth  part  of  the  air  consists  of  oxygen,  the  other 


12  INTRODUCTION 

four-fifths  of  nitrogen;  they  are  only  mixed,  not  com- 
bined. Melted  lead  has  a  strong  affinity  for  oxygen, 
which  it  takes  readily  from  the  air  in  such  proportion  as 
to  form  litharge;  gold  and  silver. at  the  heat  of  melted 
lead  do  not  combine  with  oxygen;  hence,  when  an  alloy 
of  lead,  gold,  and  silver  is  melted  in  air,  the  lead  oxidizes, 
the  gold -and  silver  do  not;  if  the  litharge  is  removed  as  it 
is  formed,  all  the  lead  is  finally  separated  from  the 
precious  metals;  this  is  the  philosophy  of  cupellation. 

When  silver  is  placed  in  nitric  acid,  it  develops  an 
affinity  for  a  compound  of  nitrogen  and  oxygen  which 
exists  in  the  acid;  gold  does  not;  hence,  when  an  alloy 
of  gold  and  silver  is  boiled  in  nitric  acid,  the  silver 
forms  a  compound  known  as  nitrate  of  silver,  which  is 
soluble  in  water,  and  if  the  acid  is  somewhat  dilute,  all 
of  the  silver  is  dissolved,  while  the  gold  remains.  From 
the  law  of  combination  in  definite  proportions  it  is  neces- 
sary that  the  acid  be  present  in  sufficient  quantity,  other- 
wise a  portion  of  the  silver  remains  unaltered;  if  more 
than  the  requisite  quantity  be  used,  the  excess  of  acid 
remains  unchanged.  In  practise,  the  alloy  must  contain 
at  least  twice  as  much  silver  as  gold,  otherwise  the 
insoluble  gold  so  envelops  the  silver  as  to  protect  it  from 
the  action  of  the  acid,  except  superficially. 

Thus  the  art  of  assaying  is  dependent  on  the  laws  of 
chemistry,  and,  although  it  may  be  practised  with  con- 
siderable success,  it  can  not  be  properly  understood 
unless  those  laws  are  studied. 

An  assay  is  an  operation  performed  on  a  known  quan- 
tity of  matter  for  the  purpose  of  ascertaining  how  much 
of  a  certain  substance  that  quantity  of  that  kind  of  mat- 
ter contains.     A  test  is  an  operation  performed  on  an 


INTRODUCTION  1 3 

indefinite  quantity  of  matter  in  order  to  ascertain  the 
character  of  that  matter,  or  to  determine  the  presence  or 
absence  of  some  particular  substance.'''  Thus,  to  test  a 
piece  of  rock  for  silver  is  merely  to  try  whether  it  con- 
tains any  silver  or  not;  to  assay  it  for  silver  is  to  find 
out  how  much  silver  is  contained  in  a  weighed  quantity, 
as  an  ounce  or  a  half  ounce,  and  thence,  by  calculation,  in 
a  ton  of  such  rock.  Testing  may  often  precede  assaying 
with  great  advantage,  because  it  enables  us  to  know  what 
kind  of  matter  we  have  to  deal  with,  and  thus  to  adapt 
our  method  so  as  to  insure  a  correct  assay;  or  it  may 
show  us  that  an  assay  would  be  useless  because  of  the 
absence  of  the  substance  sought,  or  of  its  presence  in  such 
minute  quantity  only  as  to  be  practically  worthless. 

To  avoid  confusion  hereafter,  it  is  as  well  to  mention 
that  not  only  the  act  of  assaying,  but  also  the  definite 
quantity  of  matter  operated  on,  is  called  an  assay.  Also, 
a  test  is  either  the  act  of  testing,  the  thing  tested,  or  the 
agent  by  means  of  which  a  test  is  made. 

Assays  and  tests  are  of  two  principal  classes,  the  dry, 
or  "fire,  and  the  wet,  or  humid,  assay  or  test.  In  the  dry 
way,  the  substance  under  examination  is  usually  melted 
by  heat,  with  the  addition  of  such  substances  as  may 
be  necessary  to  produce  fluidity  and  to  separate  the  par- 
ticular substance  sought  from  the  other  components. 
The  added  substances  are  called  fluxes,  reducers, 
oxidizers,  desulphurizers,  etc.,  according  to  their  func- 
tions in  the  operation.  In  the  wet  way,  the  substance,  if 
soHd,  is  acted  on  by  means  of  liquid  solvents,  such  as 

*These  definitions  are  in  accordance  with  technical  usage  rather 
than  with  etymology. 


14  INTRODUCTION 

acids,  etc.,  which  convert  it,  wholly  or  in  part,  into  a 
liquid.  By  the  addition  of  ''reagents"  to  the  liquid,  the 
substance  sought  is  separated,  or  its  presence,  and  in 
assays  its  quantity  is  determined  by  the  occurrence  of 
some  appreciable  phenomenon,  such  as  the  production  of 
a  precipitate  or  of  a  color.  For  example,  a  portion  of 
impure  bullion  is  dissolved  in  nitric  acid,  and  a  solution 
of  common  salt  is  added;  the  formation  of  a  white  pre- 
cipitate indicates  the  presence  of  silver;  this  is  a  test.*  If 
the  portion  of  bullion  be  weighed,  and  the  exact  quantity 
of  salt  required  for  the  complete  precipitation  of  the  sil- 
ver be  also  ascertained,  the  proportion  of  silver  in  the  bul- 
lion is  easily  deduced ;  this  is  an  assay.  Again,  a  substance 
supposed  to  contain  copper  is  dissolved  in  acid,  and 
ammonia  is  added;  the  production  of  a  blue  color  indi- 
cates that  copper  is  present,  and  if  the  portion  of  the 
substance  be  weighed  before  being  dissolved,  we  can 
ascertain  how  much  copper  it  contains  by  noting  how 
much  cyanide  of  potassium  it  takes  to  destroy  the  color.* 
Silver  ores  are  assayed  in  the  dry  way,  gold  ores  by  a 
combination  of  the  dry  and  the  wet  way.  By  ores  is 
here  meant  all  mineral,  rock,  or  earth  containing  the 
metal  sought  in  small  particles,  or  in  chemical  combina- 
tion. Silver  ores  very  frequently  contain  gold,  and  gold 
rarely  occurs  quite  free  from  silver.  The  ore  is  ground 
to  powder,  weighed,  and  melted  in  an  earthen  vessel 
(crucible  or  scoriHer)  with  fluxes  including  lead  in  some 

*This  must  only  be  taken  as  an  illustration.    Lead  or  mercury 
in  certain  circumstances,  would  also  give  a  white  precipitate. 

*Nickel  also  gives  a  blue  color,  and,  if  there  is  any  question, 
must  be  distinguished  by  other  tests. 


INTRODUCTION  1 5 

form;  a  glassy  mass  (slag)  and  a  lump  of  lead  (button) 
result.  The  lead  contains  the  precious  metal,  and  is 
melted  on  a  porous  support  (cupel)  in  an  oven  (muMe), 
converted  into  litharge  and  absorbed  by  the  support. 
The  gold  and  silver  remain  in  the  form  of  a  bead  on  the 
support ;  the  two  are  parted  by  boiling  the  bead  in 
nitric  acid.  The  following  described  implements  and 
material  are  used,  which  can  be  obtained  from  any  dealer 
in  chemists'  and  assayers'  supplies. 


^ 


i6 


ASSAYING    GOLD 


IMPLEMENTS. 


Iron  Mortar  and  Pestle  for  Pulveriz.ng  Ores. 
— Size  and  price  of  each:  ^  pint,  75  cents;  i  pint,  $1.00; 
I  quart,  $1.25;  Vi  gallon,  $1.75;  i  gallon,  $2.50;  1V2  gal- 
lons, $3.00;  2  gallons,  $4.75 ;  3  gallons,  $7.50.  It  is  con- 
venient to  have  several  sizes,  but  the  third  or  fourth 
size  will  answer  all  purposes.  It  is  also  convenient  to 
have  ^^ 

Bucking  board 
WITH  Muller,  20X 
24  inches,  made  of 
cast  iron;  price,  $13, 
or  a  rectangular 
grinding-plate,  7x28 
inches,  with  trun- 
nions and  bearings 
at  side,  to  discharge 
the  ground  ore  into 
a  pan  by  tilting  the 

^,jj  plate;   price,  $6.50. 

Buck's   Patent 

Amalgam  Mortar  may 
replace  plate  and  muller 
for  grinding.  Is  also 
good  for  the  assay  by 
amalgamation.  Diame- 
ter, 8^  inches;  weight 
of  mortar  and  muller,  68 
pounds;  price,  $7.50. 


AND  SILVER  ORES 


17 


\\/ 

'edge  WOOD 

Mortar.  — 

-   Price 

and 

diameter     of 

each 

:     M 

inches, 

75 

cents 

;   A'A 

inches,     90 

cents ; 

SVa 

inches. 

$1.00; 

6     inches, 

$1.25; 

6M 

inches, 

$1.50; 

7^/2 

inches. 

$1.75; 

Power  Crusher  and 
Grinder.  —  Where  many 
assays  have  to  be  made  on 
short  notice,  a  crusher  and 
grinder  run  by  power  will 
be  found  almost  indispen- 
sable. Very  good  types  of 
these  machines  are  shown 
in  the  cuts.  The  crusher 
costs  $43.00,  and  the 
grinder  $85.00. 


8^  inches,  $2.50.  Chiefly  used  for  powdering  fluxes  and 
mixing  the  assays.  One  of  these  is  sufficient,  and,  in  case 
of  need,  may  be  dispensed  with. 

Sieves. — These  should  be  eight  in  number,  respectively, 
of  2,  4,  10,  20,  40,  60,  80,  and  100  meshes  to  the  running 
inch.  They  are  formed  of  brass  wire  cloth  stretched  on 
circular  rims  of  wood  or  tin,  the  latter  being  preferable. 
The  2  and  4  mesh  sieves  should  be  about  12  inches  in 


1 8  ASSAYING    GOLD 

diameter,  respectively,  the  others  about  8  inches  in 
diameter.  It  is  very  convenient  to  have  sieves  which 
will  fit  about  two-thirds  of  the  way  into  the  sample  pans 
(see  below). 

Sample  Pans. — These  are  similar  to  cake-tins.  There 
should  be  two  sizes,  about  8  and  5  inches  in  diameter 
respectively.  The  large  diameter  of  the  8-inch  pan 
should  be  slightly  larger  than  that  of  the  8-inch  sieve 
noted  above. 

Spatulas^  etc. — Two  apothecaries'  spatulas,  the  one 
about  10,  the  other  about  6,  inches  long ;  and  a  few  large 
and  small  common  spoons  should  be  provided. 

Pulp  Scales. — These  scales  cost  $11,  will  carry  two 
ounces,  and  are  sensitive  to  one  milligram  when  new. 
A  set  of  assay  ton  weights  from  2  A.  T.  to  1-20  of  an 
A.  T.  are  very  necessary,  and  cost  $6.00;  also  a  set  of 
Weights  from  50  grams  to  i  gram,  costs  $5.00,  or  a  set 
from  50  grams  to  i-ioth  of  a  milligram,  including  all  the 
weights  necessary  for  this  scale  and  the  finer  assay  bal- 
ance to  be  described  hereafter,  may  be  bought  for  $12.00. 
Grain  weights  can  be  had  if  desired.  The  best  weights 
should  always  be  bought. 

In  addition  to  the  pulp  scales,  an  ordinary  grocer's 
scales  will  be  found  useful  for  weighing  out  large  sam- 
ples of  ore,  fluxes  in  bulk,  etc.  Cost,  including  avoirdu- 
pois weights,  about  $12.00. 

Crucibles. — Several  kinds  of  assay  crucibles  are  in 
common  use,  among  others  the  Denver  and  the  Battersea. 
They  are  made  of  fine  clay,  and  may  be  used  a  number 
of  times ;  the  most  suitable  sizes  are  the  F  and  G  Batter- 


AND   SILVER   ORES 


19 


sea,  and  the  20,  30,  and  40  gram  Denver,  costing  from 
$0.60  to  $1.20  per  dozen.  Covers  made  of  clay  cost  about 
75  cents  per  dozen,  but  for  the  country  it  is  better  to  buy 
No  2  black-lead  crucible  covers,  at  20  cents  each,  as  they 
last  very  much  longer.  They  can  be  cut  to  fit  the 
crucibles. 

Crucible  Rack. — This  is  simply  a  small  portable 
bench  with  eight  or  twelve  holes,  in  which  the  charged 
crucibles  may  be  placed  in  readiness  for  the  furnace,  thus 
avoiding  the  risk  of  upsetting,  and  enabling  a  number 
to  be  carried  at  once  from  the  mixing-table.  It  can  be 
made  by  any  one  who  can  use  carpenters'  tools. 


ScoRiFiER. — A  dish  or  cup  forrhed  of  clay.  There 
are  different  sizes,  but  that  most  used  is  2.]/^  inches  in 
diameter,  which  admits  of  two  being  placed  in  the  width 
of  a  mint-muffle.     Price,  $1.20  per  hundred. 

Roasting  Dishes. — Similar  to  scorifiers,  but  shallower, 
and  from  three  to  six  inches  wide ;  as  they  are  used  in  the 
muffle,  the  size  should  be  selected  to  suit.  Price,  $1.00  to 
$2.50  per  dozen. 


20 


ASSAYING    GOLD 


Muffle  and  Muffle 
Furnace. — The  muffle  is 
a  sort  of  small  oven,  of  a 
semi-cylindrical  form,  made 
of  clay,  open  at  the  front 
end,  closed  at  the  back,  ex 
cept  a  narrow  slit  or  two 
small  holes.  It  is  heated 
by  being  surrounded,  ex- 
cepting the  front  end,  by 
glowing  coals  in  the  muffle  furnace.  In  it,  objects  are 
exposed  to  heat  out  of  contact  with  the  fuel.  The  muffle 
furnace  is  a  sheet-iron  cylinder,  lined  with  fire  tiles,  hav- 
ing an  opening  at  the  back  to  connect  with  a  flue  or 


case  muffle  furnace 


stovepipe,  and  three  openings  at  the  front  with  doors,  the 
upper  one  corresponding  with  the  open  end  of  the  muffle, 
the  middle  one  for  introducing  fuel  under  the  muffle, 
the  lower  one  for  the  admission  of  air  and  removal  of 


AND   SILVER   ORES  21 

ashes.  Immediately  above  the  lower  door  is  a  grate.  The 
open  end  of  the  muffle  rests  in  the  upper  opening,  the 
closed  end  on  a  projection  of  the  lining,  or,  if  not  extend- 
ing across  the  furnace,  on  a  piece  of  fire-brick,  or  an 
assay  crucible.  The  top  of  the  furnace  is  closed  by  a 
movable  iron  cover.  This  furnace  uses  coke  as  a  fuel,  and 
is  the  type  of  furnace  used  very  extensively  a  few  years 
ago.  The  more  modern  coke  furnaces  are  rectangular  in 
form,  and  are  more  solidly  built.  They  are  also  con- 
structed to  burn  either  coke  or  soft  coal.  For  ordinary 
mine  work  a  furnace  burning  either  gasoline  or  oil  is 
usually  the  best  type  to  use.  The  accompanying  cuts  illus- 
trate the  combination  type  of  furnace. 

Unless  the  furnace  is  built  from  the  floor  up,  it  should 
stand  on  a  solid  table,  of  such  a  height  that  the  operator 
may  work  conveniently  in  the  muffle,  standing  or  sitting 
as  preferred.  The  top  of  the  table  should  be  protected  by 
sheet  iron,  and  large  enough  to  afford  room  for  the  assay 
mold,  cupel-tray,  tongs,  etc.  Under  the  table  may  be  a 
bin  for  fuel.  When  gasoline  is  used  as  fuel  every  precau- 
tion should  be  taken  to  avoid  explosion. 

Melting  Furnace. — Fur  fusing  the  assays  in  cruci- 
bles, also  for  melting  bullion.  The  cylindrical  muffle  fur- 
nace, described  above,  may  be  used  if  the  muffle  be 
removed,  and  the  muffle  opening  sealed.  A  standard  fur- 
nace for  making  fusions  is  the  one  shown  in  the  cut. 
Some  assayers  prefer  to  carry  on  all  furnace  operations, 
except  the  melting  of  bullion,  in  the  muffle,  and  for  this 
purpose  use  a  large  size  muffle.  Coal,  coke,  gasoline  or 
oil  may  be  used  as  fuel.  Bullion  melting  is  preferably 
done  in  a  special  furnace. 


22 


ASSAYING    GOLD 


CASE  CRUCIBLE  FURNACE 


Fire  Irons. — Comprise  a  bent  and  a  straight  poker, 
a  fire-shovel,  and  scorifier  tongs  30  inches  long;  price 


90  cents;    cupel-tongs,  26  inches  long,  price  90  cents; 


AND  SILVER  ORES  2^ 

assay  crucible  tongs,  32  inches  long,  price,  $1.00.  Other 
varieties  of  tongs  are  sometimes  used,  but  are  not  abso- 
lutely necessary. 

Assay  Molds. — Made  of  light  cast  iron.  Used  to 
pour  the  melted  assays  into  from  crucibles  or  scorifiers. 
The  conical  kind  with  four  holes  is  best  for  crucible 
assays,  and  the  shallow  kind  with  twelve  round  holes  is 
the  best  for  scorification  assays.     Price,  $1.00. 

Plyers,  Etc. — Two  pairs  of  flat-nosed  plyers,  respec- 
tively 4  and  6  inches  long;  prices,  35  and  50  cents,  used 
for  removing  beads  from  cupels.  A  7-inch  pair  of  snip- 
shears,  $1.25.  A  pair  of  scissors,  6  to  8  inches  long. 
A  pair  or  two  of  coarse  forceps,  about  6  inches  long, 
used  for  handling  lead  buttons  while  hammering  them 
etc.  A  pair  of  fine  steel  forceps,  5  inches  long,  for 
lifting  assay-weights  and  beads.  Some  use  ivory-pointed 
forceps  for  the  weights  and  weighing  capsules.  It  is 
well  to  have  a  variety,  but  the  fine  steel  "pincettes," 
rather  long  and  requiring  but  very  little  force  to  close 
them,  suit  the  author  best  for  handling  the  smaller 
weights  and  beads,  which,  with  stiff  or  coarse  forceps, 
are  apt  to  be  lost.    Prices,  50  cents  to  $1.75. 

Spirit  Lamp. — Used  for  boiling  in  test-tube.  Price, 
50  cents.  A  few  very  small  coal-oil  lamps  will  be  found 
convenient  for  various  purposes,  replacing,  for  most 
uses,  the  spirit-lamp  and  gas-jet. 

Watch  Glasses. — To  place  beads  and  other  small 
things  on. 

Hammers,  Etc. — One  steel  hammer,  about  6  ounces, 
for  large  beads,  etc.,  35  cents;  another,  about  i  pound, 
for  beating  lead  buttons  and  breaking  specimens,  etc. ; 
one  of  4  pounds,  for  breaking  coarse  rock ;  a  very  small 


24 


ASSAYING    GOLD 


one,  such  as  is  used  by  a  watchmaker,  is  useful  for 
flattening  the  smallest  beads ;  a  wooden  mallet,  a  hatchet, 
a  steel  cold-chisel  about  8  inches  long,  and  another  about 
4  inches;  a  nail-brush  for  cleaning  buttons,  and  a  tooth- 
brush for  beads ;  a  camel's-hair  pencil  and  a  flat  brush  of 
the  same,  about  two  inches  wide. 

Anvils. — One  about  3  inches  square,  with  shank  by 
which  to  fix  it  in  a  block,  and  a  flat  one  to  lay  on  a 
table;  the  first  to  beat  and  clean  lead  buttons  on,  the 
second  to  use  in  cleaning  or  flattening  small  beads  of 
precious  metal. 

Cupels  and  Cupel  Mold. — A  cupel  is  a  small,  thick 
cup,  made  of  ground  bone-ashes  pressed  in  a  mold. 
Cupels  may  be  bought  ready-made,  but  it  is  better  to 
make  them.  The  molds  are  of  brass  or  iron,  the  latter 
being  the  cheaper,  and  equally  good,  if  not  allowed 
to  rust.  At  least  two  sizes  are  required: 
The  larger  i^  or  2  inches,  the  smaller  1% 
inches  in  diameter;  prices,  $1.50  and  $2.20, 
if  of  iron.  The  cupel  is  used  in  the  sepa- 
ration of  base  from  precious  metal,  by  the 
aid  of  heat,  in  the  muffle.  To  make  a 
cupel,  moisten  the  bone-ashes  sufficiently 
to  make  them  cohere  slightly  when  com- 
pressed in  the  hand ;  fill  the  mold,  and  drive 
the  die  down  by  a  few  blows  with  a  mallet, 
and  turn  it  around,  to  smooth  the  cupel. 
The  mold,  being  bottomless,  must  be  placed 
on  something  solid,  as  the  anvil.  The  cupel 
jis  pushed  out  from  the  mold  by  means  of 
the  die;  if  this  is  very  difficult  to  do,  it  is 
because  the  ashes  are  too  dry,  or  the  die 


AND   SILVER   ORES 


25 


has  been  driven  too   forcibly;   however,   the  best  mold 
is   one   that   is    slightly   larger   at   the   bottom   than   at 
the   top.      The    cupels    are   placed    on   a    tray   to    dry— 
The   price   of  bone-ashes   is   6   cents   per   pound.     Ten 
pounds    will    make    about   200   medium-sized    cupels. 

Where  many  cupels 
are  used  the  automatic 
cupel  machine  shown  in 
the  cut  is  preferable  to 
the  single  hand  mold, 
as  hundreds  of  cupels 
of  perfect  shape,  uni- 
form in  size  and  den- 
sity may  be  turned  out 
in  a  short  space  of  time. 

Test  Tubes. — In  nests,  3  to  6  inches;  per  dozen,  35 
cents ;  single,  3,  4,  5,  6,  7,  8  inches ;  per  dozen,  20  to 
60  cents.     Made  of  glass;  used  for  boiHng  acids,  etc. 

Test-Tube  Rack. — To  set  the  tubes  in;  also  has  pins 
for  draining  tubes  by  inversion;  price,  75  cents. 


Test-Tube  Holder. — Wooden  tongs,  with  round 
jaws,  usually  lined  with  cork.  To  hold  the  test-tubes 
while  hot.  (o 


Dry  Cups  (Annealing-cups). — Small    crucibles,    very 


2.^  ASSAYING    GOLD 

thin;  made  of  clay,  unglazed.  Used  for  drying  and 
annealing  the  gold  from  an  assay.  Price,  for  best, 
$i.oo  per  dozen. 

Porcelain  Capsules. — Small  glazed  crucibles,  very 
thin.  Used  for  partings,  and  for  drying  and  annealing 
the  gold- from  an  assay.    Price,  for  best,  $i.oo  per  dozen. 

Parting  Flasks. — Special  form,  called  the 
Kennedy,  are  best.  Used  for  partings.  Price, 
$1.25  per  dozen. 

Matrass  Flasks. — Used  for  parting  in  the 
gold  bullion  assay.  Size,  3  oz.  Made  of  glass.  Price, 
$1.50  per  dozen. 

Water  Bath. — A  dish  having  a  cover  consisting  of 
a  series  of  rings.  By  removing  more  or  less  of  the 
rings,  a  suitable-sized  opening  is  made  in  which  to  rest 
a  vessel  to  be  heated  by  boiling  water.  Used  for  drying 
ore,  etc.,  at  heat  of  boiling  water. 

Lens,  or  magnifying  glass,  for  examining  small  beads 
of  metal  to  see  if  they  are  clean;  also  for  inspection 
of  minerals. 

Cupel  Trays. — Made  of  cast  iron,  8x8  inches,  to 
carry  cupels   on;    16  holes,   $1.20. 

Coal-oil  Stove. — This  is  a  great  convenience  in  a 
country  office  where  gas  can  not  be  had.  A  pan  con- 
taining some  sand,  placed  on  the  stove,  forms  a  sand- 
hath  for  heating  liquids  in  glass  or  porcelain  vessels, 
drying  samples  on,  etc. 


AND   SILVER   ORES 


27 


Glass  Funnels. — Price  (each)  :  Square  2 
inches,  10  cents;  3  inches,  15  cents;  4  inches, 
20  cents;  5>^  inches,  30  cents. 

Filter  Paper. — Used  for  filtering  Hquids 
to  separate  soHd  matter.  Cut  in  circular  form, 
folded  twice  across,  and  applied  as  a  lining 
to  a  funnel,  into  which  the  liquid  is  then  poured.  The 
clippings  of  the  paper  are  excellent  for  cleaning  glass- 
ware; 4  inches,  18  cents;  6  inches,  25  cents;  8  inches, 
30  cents;  10  inches,  40  cents.  Swedish  is  the  best; 
cheap  gray  answers  for  filtering  rain-water  for  assay- 
ing,   and    for    some    other   purposes. 


Filter  Stand. — Also  used  in  heating  a 
vessel  over  a  lamp.  Price,  with  2  rings,  60 
cents;   3  rings,  75  cents;   4  rings,  $1.00. 

Washing  Bottle. — Used  in  washing 
objects  in  vessels  or  on  filters.  The  bottle 
is  to  be  nearly  filled  with  water.  On  blow- 
ing into  one  of  the  tubes,  a  jet  is  expelled 
from  the  other.     Price,  50  cents. 


Assay  Balance. — This*  is  the  most  costly  and  delicate 
apparatus  used  by  the  assayer.  The  upper  cut  represents 
a  good  balance,  costing  $95.00;  the  lower  cut,  an 
extremely  delicate  balance,  suitable  for  umpire  and 
other  fine  work,  costing  $250.00.  It  is  by  no  means 
advisable  to  buy  the  cheapest  grades,  unless  in  case 
of  absolute  necessity,  and,  if  possible,  one  of  the  finer 
balances  should  be  obtained  for  assay  offices  where  a 
check  with  smelter  or  other  assayer  is  required.    A  fair 


28 


ASSAYING    GOLD 


balance  can  be  had  for  $65.00,  but  much  below  this 
price  they  can  have  no  apparatus  for  riders,  which  is 
usually  considered  a  defect  where  rapid  work  is  essen- 
tial. But  in  first  using  any  balance,  it  is  far  safer  to 
weigh  by  the  methods  of  deflections,  as  explained  later. 
A  good  balance  should  take  at  least  one  gram  in  each 


pan,  and  should  be  sensitive  to  i-iooth  miUigram.  It 
should  be  provided  with  a  set  of  platinum  weights  from 
one-tenth  milligram  to  one  gram,  and  a  beam-rider  for 
weighing  fractions  of  a  milligram.  It  is  enclosed  in  a 
glazed  case,  the  front  of  which  is  arranged  to  open  by 
an  upward  sliding  panel,  counterpoised  like  a  window- 


AND   SILVER  ORES  29 

sash,  by  means  of  cords  and  weights.  The  case  is  sup- 
ported by  legs  which  are  adjustable  in  length  by  screw- 
ing into  brass  sockets.     Within  the  case  are  two  spirit- 


levels  at  a  right  angle  the  one  to  the  other.  The  beam  is 
poised  on  knife-edges  of  agate,  resting  on  agate  plates. 
The  pans  are  suspended  by  means  of  stirrups  with  agate 
plates,  resting  on  knife-edges  or  points  fixed  to  the  beam. 
A  slender  finger  of  steel  descends  or  rises  from  the  center 
of  the  beam,  indicating  its  slightest  movement  upon  a 
graduated  arc  of  ivory.  The  distance  from  the  center  of 
the  beam  to  the  points  on  which  the  pans  are  suspended  is 
divided  into  a  certain  number  of  equal  parts,  marked  on 
the  front  of  the  beam.  These  divisions  are  subdivided  by 
shorter  marks.  A  brass  bar  extends  across  the  case 
behind  the  beam ;  it  supports  a  sliding  rod  which  extends 
to  the  outside  of  the  case,  and  terminates  in  a  milled 
head.  The  sliding  rod  is  called  a  carrier,  and  is  fur- 
nished at  its  inner  end  with  an  arm  extending  at  a  right 


30  ASSAYING    GOLD 

angle  to  the  rod,  over  the  beam.  On  the  arm  hangs  a 
small  bifurcate  weight,  formed  of  fine  wire  in  such  a 
manner  that  it  can  be  placed  astride  upon  the  beam, 
like  a  man  on  horseback.  By  means  of  the  carrier, 
operated  from  without  the  case,  the  rider  is  placed  upon 
the  beam  at  any  desired  point,  or  removed  at  pleasure. 
The  rider  acts  upon  the  principle  of  the  pea  of  a  steel- 
yard, and  is  used  to  complete  the  weighing  of  an  object 
on  the  balance,  without  the  necessity  of  opening  the 
case. 

In  order  that  the  instrument  may  not  be  injured  by 
the  weight  of  the  beam  and  pans  resting  constantly 
on  the  delicate  points  of  suspension,  nor  by  any  shock 
occasioned  by  the  placing  or  removal  of  weights  or 
other  objects  during  the  operation  of  weighing,  an 
apparatus  is  provided  by  which,  on  turning  a  key  or 
milled  head,  the  beam  is  lifted  off  its  bearings,  and  two 
small  tables  rise  under  the  pans  and  receive  their 
weight;  the  stems  of  these  tables  are  screws,  working  in 
brass  sockets,  by  which  the  height  of  the  table  is  adjusted 
by  turning  it  around. 

Above  or  below  the  center  of  the  beam,  working  on 
a  screw-thread  on  the  index,  or  on  a  separate  slender 
stem,  is  a  little  ball  or  "cheese"  of  brass,  called  a  gravity- 
bob  ;  by  screwing  this  upwar,d  or  downward,  the  center 
of  gravity  of  the  beam  is  raised  or  lowered,  and  its  sen- 
sitiveness increased  or  diminished.  Around  this  stem  is 
twined  a  slender  bit  of  wire,  with  one  end  projecting  in 
front;  by  turning  this  projecting  end  a  little  to  one  side 
or  the  other,  the  beam  may  be  balanced. 

The  balance  should  be  supported  firmly,  preferably 
on  strong  posts  running  into  the  ground  and  not  touch- 


AND   SILVER   ORES  3 1 

ing  the  floors.  The  idea  of  this  is  to  avoid  the  vibra- 
tions of  the  building,  which  interfere  with  accurate 
weighing.  This  is  sometimes  accomplished  by  placing 
the  balance  on  a  shelf  attached  to  a  brick  wall  or  chim- 
ney. The  legs  of  the  scales  should  rest  on  pieces  of 
plate-glass,  to  prevent  sinking.  In  a  moist  climate,  an 
open  vessel  containing  some  strong  sulphuric  acid  or 
quicklime  should  remain  constantly  within  the  case.  In 
a  very  hot  and  dry  climate,  makers  recommend  keep- 
ing a  shallow  dish  containing  water  on  the  table  under 
the  balance.  The  acid  or  lime  is  to  prevent  rusting,  by 
absorbing  the  moisture  of  the  air.  The  dish  of  water 
is  to  prevent  warping  of  the  woodwork  by  too  great 
dryness.  A  light  cover  of  canvas,  paper,  or  wood,  in 
the  form  of  a  box,  should  be  placed  over  the  case  when 
the  balance  is  not  in  use,  in  order  to  exclude  dust.  The 
balance  should  not  be  in  the  same  room  in  which  ore  is 
pulverized,  or  acid  boiled,  nor  in  which  is  any  furnace. 
The  weights  used  with  this  balance  are  either  grains  or 
grams,  corresponding  with  those  used  in  weighing  the 
ore  for  the  assay.  If  grains,  the  set  is  from  ten  grains 
to  one-hundredth  grain;  if  grams,  from  one  gram  to 
one  milligram.  In  either  case,  the  smallest  is  one- 
thousandth  of  the  largest,  and  is  considered  as  the  unit, 
or  one,  of  the  system.  (Where  riders  are  not  used,  the 
set  includes  smaller  weights,  but  these  are  regarded  as 
fractions  of  the  unit.)  The  weight  of  the  rider  must 
agree  with  the  other  weights,  and  with  the  number  of 
divisions  on  the  beam ;  in  some  cases  it  weighs  ten  units 
of  the  set,  and  the  beam  has  ten  principal  divisions  on 
one  or  on  each  of  the  arms,  and  fifty  subdivisions;  in 
others    the    beam    has    twelve    divisions,    and  the    rider 


32  ASSAYING    GOLD 

weighs  twelve  units;  in  either  case,  the  rider,  if  placed 
on  the  mark  i,  will  counterbalance  one  unit  of  the  set  on 
the  opposite  pan,  and  accordingly  for  the  other  marks. 
On  a  subdivision  it  is  equal  to  a  corresponding  fraction 
in  addition  to  the  preceding  whole  number.  Some 
beams,  however,  are  divided  and  numbered  in  twenty 
parts,  and  the  rider  weighs  two  units;  its  value  on  i  is 
then  one-tenth  of  a  unit;  on  2,  two-tenths,  etc. 

The  weight  of  a  bead  or  piece  of  precious  metal  ob- 
tained by  an  assay  is  usually  "reported"  in  units  of  the 
set,  not  in  grains  or  grams;  thus  a  bead  weighing  1.156 
grains  is  reported  as  11 5.6;  one  weighing  0.0765  grams 
is  reported  as  76.5.  One  gram,  or  ten  grains,  is  there- 
fore called  1,000.  In  gold  bullion  assaying  the  set  is 
generally  one-half  gram  equals  1,000,  consequently  one- 
half  milligram  is  the  unit,  and  the  rider  weighs  ten  of 
these  units  for  a  scale  with  100  divisions,  or  two  units 
for  a  beam  with  20  divisions. 

The  balance  is  adjusted  and  tested  as  follows:  The 
case  is  accurately  leveled,  first  in  one  direction,  as  from 
left  to  right,  and  then  in  the  other,  as  from  front  to  back, 
by  means  of,  the  spirit-levels  and  the  adjustable  legs. 
The  beam  is  then  placed,  not  resting  on  the  knife-edge, 
but  on  the  extra  supports  which  are  thrown  up  for  the 
purpose.  The  pans  are  then  suspended  from  the  ends  of 
the  beam.  On  each  stirrup  will  be  found  a  mark;  near 
the  corresponding  end  of  the  beam  a  similar  mark.  The 
marks  on  the  stirrups  are  placed  toward  the  front,  and 
each  stirrup  is  hung  on  its  proper  end  of  the  beam.  The 
tables  under  the  pans  are  adjusted  so  as  to  take  the 
weight  of  the  pans  without  lifting  the  stirrups  off  the 
beam.     The  supports  are  then  lowered,  and  the  index 


AND   SILVER  ORES  33 

observed;  it  should  swing  slowly  and  steadily  from  side 
to  side,  and,  on  coming  to  rest  should  point  exactly  to 
zero,  or  the  center  of  the  ivory  arc.  If  the  index  swings 
unequally,  and  stops  finally  on  one  side  of  the  zero,  the 
little  wire  which  projects  in  front  of  the  center  of  the 
beam  must  be  carefully  moved,  and  the  trial  repeated, 
and  so  on  until  perfect  equilibrium  is  attained.  The 
small  capsules  or  dishes  which  accompany  the  balance 
are  then  placed,  one  on  each  pan ;  they  must  balance  each 
other  exactly,  or  else  equilibrium  must  again  be  produced 
by  means  of  the  wire,  and  each  capsule  be  thenceforward 
used  invariably  on  the  same  side. 

It  may  be  here  remarked,  once  for  all,  that  nothing 
must  be  placed  on  the  pans,  nor  in  the  capsules,  while 
they  are  on  the  pans,  nor  must  the  beam  or  pans 
be  touched  in  any  way  until  the  extra  supports  are 
raised;  neither  must  the  capsules  or  the  weights  be 
touched  by  the  fingers.  If  it  is  necessary  to  clean  the 
capsules,  a  fine  soft  handkerchief  should  be  used.  Dust 
is  removed  from  the  beam  and  other  parts  by  means  of 
a  camel's-hair  pencil.  The  weights  and  capsules  are 
moved  by  means  of  forceps.  Occasionally  the  balance 
may  be  dismounted,  and  the  parts  cleaned  by  rubbing 
with  a  piece  of  soft  chamois  leather,  and,  if  necessary,  a 
very  little  fine  coal-oil. 

The  balance  having  been  so  far  adjusted,  must  be 
tested.  The  weight  i  of  the  set  should  cause  the  index 
or  pointer  to  swing  to  or  near  ten  divisions  from  zero, 
and  one-tenth,  obtained  by  the  riders,  should  cause  a 
deviation  of  one  division.  The  deviations  should  be  the 
same  on  either  side  with  the  same  excess  of  weight, 
otherwise  the  balance  is  badly  made.     The  sensitiveness 


34  ASSAYING    GOLD 

can  be  increased  to  the  required  degree  by  screwing  the 
gravity-bob  upward,  but  if  the  bob  is  raised  too  high,  the 
beam  will  fall  on  either  side,  and  so  remain,  and  no 
weighing  can  be  done.  Some  scales  are  not  provided 
with  a  gravity-bob,  being  adjusted  once  for  all  by  the 
maker.  In  raising  the  supports,  advantage  should  be 
taken,  whenever  possible,  of  the  moment  when  the  swing- 
brings  the  pointer  to  zero,  when  they  may  be  raised 
quickly;  if  this  can  not  be  done,  the  supports  must  be 
raised  very  gently.  In  releasing  the  beam,  the  support 
should  be  lowered  at  first  very  slightly,  until  it  is  seen 
that  the  beam  is  near  enough  to  equilibrium  to  swing 
both  ways,  then  promptly  to  the  full  extent;  but  if  the 
load  on  one  side  preponderates  so  much  as  that  the  beam 
will  evidently  fall  on  that  side  too  far  to  swing  freely,  the 
lowering  of  the  support  must  not  be  continued,  but,  on 
the  contrary,  it  must  be  raised  again,  and  a  nearer  ap- 
proach to  equilibrium  established.  These  cares  are  nec- 
esssary  to  prevent  shocks  which  would  injure  the  delicate 
bearings. 

The  next  point  is  to  ascertain  if  the  two  arms  of  the 
beam  are  of  equal  length.  This  is  done  by  placing 
counterpoising  loads  on  the  pans,  and  changing  the  loads 
from  one  pan  to  the  other;  if  the  exchange  of  loads 
produces  no  disturbance  of  the  equilibrium,  that  is,  if  an 
object  weighs  the  same  on  either  end  of  the  beam,  the 
arms  are  of  equal  length.  In  first-class  balances  there 
is  not  much  likelihood  of  a  defect  in  this  respect,  the  rec- 
tification of  which  is  a  work  of  such  delicacy  that  a  per- 
son who  is  not  familiar  with  such  work  should  not  at- 
tempt it. 

It  must  be  understood  that  the  condition  of  equilib- 


AND   SILVER   ORES  35 

rium,  and  equality  in  the  length  of  the  arms  are  dis- 
tinct; either  may  exist  without  the  other.  The  beam 
must  be  balanced  by  some  means  before  any  weighing  is 
undertaken;  if  it  is  then  found  that  the  arms  are  of  un- 
equal length,  by  testing  as  above,  recourse  must  be  had 
to  double  zveighing,  in  order  to  obtain  the  true  weight  of 
an  object.  The  best  method  of  double  weighing,  called 
the  method  by  substitution,  is  to  place  the  i,ooo  weight 
on  one  pan  of  the'  balance,  and  counterpoise  it  by  a 
special  weight  made  for  the  purpose,  of  any  suitable  ma- 
terial, as  gold  or  platinum ;  the  weight  is  then  removed, 
the  object  to  be  weighed  is  put  in  its  place,  and  smaller 
weights  added  until  the  counterpoise  is  balanced;  the 
difference  between  the  i,ooo  weight  and  the  sum  of  the 
weights  required  to  restore  the  equilibrium  is  the  weight 
of  the  object. 

It  may  be  here  observed  that  if,  as  in  assaying,  only 
relative "  weights  are  required,  the  inequality  in  length 
of  the  arms  is  of  no  consequence,  provided  that  all  the 
weighings  of  the  assay  are  made  on  the  same  pan  of  the 
same  balance,  because  then  they  are  all  affected  in  the 
same  ratio,  and  the  relative  proportions  remain  correct. 
This  is  easily  done  in  bullion  assays,  and  may  be  so  in 
ore  assays,  if  a  chemical  balance  is  used  which  is  at  once 
large  and  strong  enough  to  weigh  the  ore,  and  delicate 
enough  to  weigh  the  gold  and  silver  obtained ;  but  where, 
as  in  general,  the  ore  for  an  assay  is  weighed  on  one 
balance  and  the  precious  metal  on  another,  double  weigh- 
ing becomes  necessary,  if  the  arms  of  either  are  unequal. 

Another  method  of  double  weighing  is  by  reversal. 
The  object  is  weighed  first  on  one  pan  and  then  on  the 
other,  and  the  sum  of  the  apparent  weights  divided  by 


36  ASSAYING    GOLD 

two  is  taken  as  the  true  weight;  this  is  not  strictly  cor- 
rect. The  true  weight  is  the  square  root  of  the  product 
of  the  apparent  weights,  but  if  their  difference  is  small, 
the  error  is  inappreciable.  When  it  is  required  to  weigh 
off  a  certain  quantity  of  a  substance  on  such  a  balance, 
the  weight  corresponding  to  that  quantity  is  placed  on 
one  of  the  pans  and  counterpoised  by  any  means;  the 
weight  is  then  removed,  and  replaced  by  the  substance; 
the  quantity  of  the  latter  required  to  restore  equilibrium 
is  equal  to  the  weight  which  it  replaces. 

A  careful  assayer  will  also  test  the  weights  in  order 
to  ascertain  if  they  agree  among  themselves.  It  is  not 
important  that  they  be  absolutely  standard  but  only  that 
they  are  correctly  proportioned  the  one  to  the  other,  for 
an  assay  is  simply  a  question  of  the  proportionate  weight 
of  precious  metal,  as  compared  with  the  weight  of  ore 
from  which  it  is  extracted,  hence  the  weights  used  in 
weighing  the  ore  for  the  assay  must  accord  with  those 
used  for  weighing  the  metal  obtained,  and  all  must  agree 
among  themselves  according  to  their  marks.  If  1,000 
parts  by  weight  of  ore,  no  matter  what  kind  of  weights 
is  used,  contain  one  such  part  by  weight  of  silver,  it  is 
clear  that  1,000  pounds  of  similar  ore  contain  one  pound 
of  silver.  The  proportion  once  ascertained,  it  is  easy  to 
deduce  the  absolute  quantity  of  metal  in  a  ton  of  ore. 
The  1,000  assay  weight  should  equal  the  400,  300,  200 
and  100  assay  weights  together ;  the  400  should  equal  the 
300  and  100,  and  so  on  down  to  the  smallest.  The  ore 
weights  must  be  checked  in  a  similar  manner  on  the 
pulp  scales,  working  from  one  of  the  smaller  of  them, 
after  that  has  been  found  to  agree  with  the  similar  assay 
weight.     In  case  the  ore  weights  are  not  a  complete  set. 


AND   SILVER   ORES  37 

the  assay  weights  may  be  combined,  and  a  quantity  of 
granulated  lead  may  be  weighed  on  the  assay  balance 
as  many  times  as  may  be  necessary  to  make  a  quantity 
equal  to  the  ore  weight  as  marked,  and  placed  on  one 
pan  of  the  pulp  scales;  the  ore  weight  being  placed  on 
the  other  pan,  it  will  be  seen  whether  the  two  agree  or 
not.  The  riders  may  also  be  tested  by  weighing  them 
against  the  ten  assay  weight,  if  the  beam  has  ten  main 
divisions,  or  against  the  ten  and  two,  if  it  has  twelve 
divisions,  etc.,  and  the  divisions  on  the  beam  by  placing 
the  rider  on  each  mark  in  succession,  and  the  correspond- 
ing weight  on  the  opposite  pan.'-' 

*The  implements,  apparatus,  etc.,  illustrated  herein  can  be 
bought  from  F.  W.  Braun  &  Co.,  San  Francisco  and  Los 
Angeles,  Cal. 


38  ASSAYING    GOLD 


MATERIALS. 


Litharge  (lead  oxide,  PbO). — Twelve  to  fifteen  cents 
per  pound;  composed  of  lead  and  oxygen;  a  flux,  an 
oxidizer,  a  desulphurizer,  and  a  source  of  lead. 

A  flux,  because  it  causes  refractory  substances  to  melt 
at  a  comparatively  moderate  heat.  It  fluxes  most  rocks, 
earths,  and  metal  oxides,  hence  is  very  destructive  to  the 
crucible  if  used  in  excess,  for  which  reason  other  sub- 
stances are  used  instead,  to  a  certain  extent. 

An  oxidizer,  because  it  gives  up  oxygen  to  combustible 
or  oxidizable  substances,  causing  them  to  burn,  or  be- 
come oxidized;  it  thus  burns  sulphurets  in  an  assay.  It 
is  a  common  error  to  suppose  that  the  substances  thus 
burned  are  necessarily  burned  out.  On  the  contrary, 
while  some  are  burned  out,  others  remain  in  the  assay, 
but  their  condition  is  changed ;  they  are  converted  into 
the  respective  products  of  their  combustion. 

A  desulphurizer,  because  it  burns  the  sulphur  of  sul- 
phurets, as  well  as  oxidizing  their  metals,  except  lead 
and  noble  metals. 

A  source  of  lead,  because  when  it  gives  up  its  oxy- 
gen, the  lead  remains  in  metallic  state. 

To  prove  these  things,  melt  some  litharge  in  a  cru- 
cible, and  throw  in  about  one-fiftieth  as  much  iron  sul- 
phurets in  powder.  A  piece  of  lead  will  be  gotten  from 
a  portion  of  the  litharge;  the  rest  fluxes  the  iron  oxide 
made  by  burning  the  iron   sulphuret.     Or,   instead   of 


AND   SILVER   ORES  39 

iron  sulphuret,  throw   in  some  galena;   the  sulphur  of 
the  galena  will  be  burned,  and  lead  will  remain. 

Soda  (sodium  bi-carbonate,  NaHCOg). — Six  cents  per 
pound;   a  flux,  an  oxidizer,  and  a  desulphurizer. 

Fluxes  quartz,  quartzose  rocks,  and  some  metal  oxides. 
Oxidizes  some  metals.  Desulphurizes  galena  and  some 
other  sulphurets,  especially  if  charcoal  or  flour  is  added ; 
the  sodium  combines  with  the  sulphur.  Mix  some  pow- 
dered quartz  with  thrice  its  weight  of  soda,  and  heat  to 
bright  redness  in  a  crucible;  the  result  will  be  a  glass 
which  can  be  poured  out.  Melt  some  soda  with  iron 
filings,  and  dissolve  in  water ;  the  iron  will  be  found  to  be 
oxidized.  Melt  some  galena  with  Several  times  its  weight 
of  soda ;  lead  will  be  got. 

Borax  (sodium  biborate,  l>l?i^BJ^.^,  loH^O). — Ten 
cents  per  pound  ;   a  flux. 

Fluxes  clay;  lime,  magnesia,  slate,  etc.,  and  metal 
oxides  generally ;  also  quartz,  but  not  so  well  as  soda. 
Melt  some,  and  add  a  little  powdered  ore  containing  sul- 
phurets;  the  gangue  (rock,  etc.)  will  be  dissolved;  the 
sulphurets  will  be  found  at  the  bottom. 

Borax  swells  when  heated,  giving  off  water;  this  is 
sometimes  inconvenient  in  an  assay.  To  prepare  it  for 
use,  heat  it  gently  in  an  iron  pan  until  it  swells  no  more, 
then  cool  and  grind  it;  it  should  not  be  melted,  as  that 
makes  it  hard  to  grind ;  it  should  be  kept  in  a  close 
vessel  if  in  a  moist  climate.  Sometimes  it  is  used  un- 
dried  in  lumps.  Borax  may  be  bought  already  ground, 
and  is  commonly  used  in  this  condition,  not  being  dried. 

Carbonate  of  Potassium    (K2CO3). — This  acts  the 


40  ASSAYING    GOLD 

.same  as  sodium  carbonate,  with  which  it  is  frequently 
mixed.     It  is  valuable  in  fusing  lead  ores. 

Borax  Glass. — Forty  cents  per  pound;    a  flux,  same 

as  borax,  but  does  not  swell.     To  prepare  it,  melt  some 

borax,  cast  it  in  thin  plates,  and  grind  to  powder.  One 
part  by  weight  is  equal  to  two  of  undried  borax. 

Glass  (common  bottle  or  window). — A  flux.  Acts 
similarly  to  borax,  but  is  less  easily  melted.  Used  in 
assays  containing  much  lime,  clay,  etc. ;  not  with  quartz- 
ose  ores.  Also  useful  in  assays  made  with  much  niter 
or  litharge,  to  protect  the  pot.  A  quartzose  ore  fluxed 
with  soda  makes  glass,  which  then  fluxes  the  metal  ox- 
ides, lime,  etc.,  in  the  pre.  Sometimes  used  in  pieces,  but 
is  better  ground.  Heating  to  redness  and  quenching  in 
water  facilitates  grinding. 

Niter  (potassium  nitrate,  KNO3). — Fifteen  cents  per 
pound.  First  a  powerful  oxidizer,  then  a  flux ;  also  a  de- 
sulphurizer.  When  heated  gives  off  a  large  quantity  of 
oxygen,  leaving  potash,  which  is  a  flux  of  much  the  same 
nature  as  soda.  Niter  is  a  desulphurizer  in  two  ways : 
First,  by  giving  off  oxygen  to  burn  sulphur;  second,  by 
the  potassium  combining  with  sulphur,  as  the  sodium  of 
soda  does;  but  if  enough  niter  be  used,  all  the  sulphur 
is  burned,  being  converted  into  sulphuric  anhydride. 
Niter  can  oxidize  all  metals  except  gold  and  some  of  the 
platinum  group.  Used  to  counteract  the  effect  of  too 
great  a  quantity  of  sulphurets  in  an  assay,  which  it  does 
by  burning  a  part  of  them,  which  would  otherwise  pro- 
duce too  much  lead  from  the  litharge. 

Sulphur. — Six  cents  per  pound.  A  sulphurizer. 
Used  in  a  certain  class  of  assay  in  certain  cases  to  pre- 


AND   SILVER   ORES  4I 

vent  copper  from  entering  the  lead  button,  which  it  does 
by  converting  the  copper  into  a  sulphuret. 

Iron  (nails  or  wire,  Fe). — A  desulphurizer  for  galena 
and  for  compounds  of  silver  with  sulphur,  but  not  for 
other  metal  sulphurets,  as  copper,  zinc,  etc."^  Used  in 
a  certain  class  of  assays  to  free  lead  and  precious  metals 
from  sulphur.  Can  also  combine  with  arsenic,  keeping 
that  out  of  the  lead.  Iron  reduces  litharge  to  lead,  itself 
being  burned,  but  it  is  not  used  for  that  purpose.  Melt 
some  borax,  add  some  galena  and  a  large  nail ;  lead  will 
be  got. 

Salt  (sodium  chloride,  NaCl). — Used  because  it  be- 
comes very  fluid,  floats  on  the  assay,  and  serves  as  a 
cover  to  exclude  air  and  furnace-gases ;  also  to  wash  the 
side  of  the  pot;  it  must  be  dried  and  ground.  If  mixed 
with  the  assay,  it  tends  to  prevent  overflowing  by  be- 
coming fluid  at  a  comparatively  low  temperature,  thus 
facilitating  the  escape  of  gases. 

Sheet  Lead. — Twenty-five  cents  per  pound.  Tea  lead 
will  answer.    Must  contain  no  gold. 

Granulated  Lead  (Pb). — Fifteen  cents  per  pound. 
Used  in  the  assay  by  scorification,  in  which  the  litharge 
required  as  a  flux  is  produced  from  lead,  instead  of 
lead  being  produced  from  litharge,  as  in  the  crucible 
assay. 

Flour. — Used  in  the  crucible  to  produce  lead   from 

*Iron  can  take  all  the  sulphur  from  lead  or  silver  com- 
pounds, setting  .those  metals  free.  It  only  takes  a  part  of 
the  sulphur  from  pyrites,  sulphuret  of  copper,  and  some 
other  sulphurets,  leaving  the  metals  still  combined  with  a  part 
in  the  form  of  matte. 


42  ASSAYING    GOLD 

litharge.  One  part  by  weight  of  flour  generally  pro- 
duces fifteen  parts  of  lead,  but  this  varies  a  little,  accord- 
ing to  the  quality  of  the  flour  and  of  the  litharge.  Pow- 
dered charcoal  is  also  used  for  this  purpose ;  one  part  of 
charcoal  produces  about  thirty  parts  of  lead.  Any  sub- 
stance which  produces  lead  from  litharge  is  a  reducer. 

Argol  (KHC4H4O6). — Commercial  bitartrate  of  pot- 
ash. This  acts  as  a  reducing  agent,  and  also  as  a  basic 
flux.    One  part  of  argol  produces  about  9  grams  of  lead. 

Test  Silver. — Two  dollars  and  fifty  cents  per  ounce. 
1,000  fine.  Used  in  the  gold  assay.  Must  contain  no 
gold. 

Acids. — Nitric,  HNO^,  forty  cents  per  pound  in 
seven-pound  lots;  hydrochloric,  HCl  (muriatic),  and 
sulphuric,  H2SO4,  acids.  The  first  only  is  strictly  neces- 
sary; it  must  contain  no  chlorine.  The  others  are  con- 
venient for  certain  tests. 

Distilled  Water,  HoO. — Filtered  rain-water  will  an- 
swer; common  spring-water  is  often  used;  it  must  con- 
tain no  chlorine.  Used  in  the  gold  assay  for  washing  the 
gold. 

For  the  sake  of  brevity,  only  the  more  important  prop- 
erties of  the  foregoing  substances  have  been  mentioned. 

The  water  and  nitric  acid  may  be  tested  for  chlorine 
thus :  Dissolve  a  few  grains  of  silver  by  boiling  in  rather 
dilute  nitric  acid,  add  a  drop  or  two  of  the  solution  to 
the  suspected  water  or  acid  in  9.  clear  glass  vessel,  and 
observe  if  it  produces  a  white  cloud  or  a  milkiness;  if  it 
does  so  in  the  water,  that  will  better  be  rejected,  but 
not  so  the  acid.  Stir  the  acid  with  a  glass  rod,  or,  if  in 
a  bottle,  which  is  best,  shake  the  bottle  well,  then  let  it 


AND   SILVER   ORES  43 

stand  until  the  acid  is  clear,  and  add  another  drop  of 
silver  solution,  and  so  on  until  no  further  milkiness  is 
produced ;  then,  after  settling,  pour  the  clear  acid  off 
into  another  bottle  for  use.  The  water  may  be  treated 
in  the  same  way  in  case  of  need.  In  a  mill,  good  con- 
densed water  can  be  got  from  the  boiler  (not  from  the 
exhaust  of  the  engine). 

The  assays  of  litharge  and  lead  for  gold  and  silver  will 
be  found  in  their  appropriate  places.  Litharge  and  lead 
always  contain  some  silver,  and  this  is  allowed  for  in  the 
assays  of  ore ;  if  they  contain  gold  also,  they  are  unfit  for 
use. 

Any  silver  that  is  nearly  pure  will  answer  for  test 
silver,  if  it  contains  no  gold ;  to  test  it,  dissolve  about  one- 
half  gram  in  nitric  acid;  it  should  leave  no  residue. 

All  the  fluxes  should  be  ground  to  powder.  The  prices 
given  are  those  at  present  prevailing  and  are  liable  to 
change;  a  discount  is  always  allowed  on  considerable 
purchases. 


44  ASSAYING    GOLD 


THE  ASSAY  OFFICE. 


The  office  should  properly  consist  of  three  adjoining 
rooms:  One  in  which  the  pulverizing,  fire  work,  and 
other  rough  and  dirty  work  is  done ;  another  in  which  to 
weigh  and  mix  the  assay,  boil  the  acid  in  separating  gold 
from  silver,  and  keep  the  fluxes,  acids,  crucibles,  spare 
muffles,  etc. ;  and  a  third  in  which  to  keep  the  assay- 
balance,  books,  and  papers ;  but  it  often  happens  that  only 
one  room  can  be  had,  and  all  that  can  be  done  is  to  make 
the  best  of  it.  The  muffle-furnace  should  be  so  placed 
that  when  the  operator  works  in  the  muffle  the  light 
comes  from  behind  him.  The  sun  should  not  shine  into 
the  room.  There  should  be  a  strong  bench  or  table  on 
which  to  use  the  pulverizing  implements,  and  it  is  better 
if  it  rest  on  the  ground,  and  not  on  the  floor,  so  that  the 
entire  office  may  not  be  shaken  by  the  blows  of  the  pestle 
or  hammer.  A  block,  similar  to  that  used  by  a  butcher, 
will  answer  well  instead  of  a  table.  Another  table  on 
which  to  place  the  pulp-scales  and  mix  the  assays  will  be 
required.  Above  this  table,  or  in  any  convenient  situa- 
tion, should  be  shelves  for  packages  of  material,  and 
upon  the  table,  which  should  be  of  a  good  size,  a  long 
box,  divided  across  into  five  or  six  compartments,  about 
six  inches  square  and  deep,  and  lowest  in  front,  with  a 
hinged  cover,  to  contain  the  several  fluxes,  etc.,*  also 
the  test-tubes  in  their  rack,  the  spirit-lamp,  smaller  anvil, 
etc. 

*In  a  damp  climate  it  is  better  to  keep  the  fluxes  in  separate 
covered  vessels  of  tinware  or  glass. 


AND   SILVER   ORES  45 

The  assay-scales  should  be  so  placed  that  the  light  from 
a  window  may  strike  them  obliquely  over  the  left  shoul- 
der of  the  operator  while  seated  at  his  work;  it  is  still 
better  to  have  light  from  two  sources  obliquely  behind 
the  operator,  in  order  that  no  shadows  may  be  cast  on 
the  balance,  either  by  the  person  or  by  the  case-frame.f 

The  sun  should  not  be  allowed  to  shine  on  the  balance 
at  any  time.  The  table  on  which  the  balance  stands 
should  be  reserved  for  that  purpose  only;  there  should 
be  another  on  which  to  work  with  pen  and  paper. 

tif  there  is  any  difficulty  in  getting  light  from  the  proper  direc- 
tion, a  mirror  may  be  so  placed  as  to  reflect  on  the  balance.  A 
hand-mirror  is  sometimes  useful  to  throw  light  on  the  beam 
while  reading  the  indications  of  the  rider. 


46  ASSAYING    GOLD 


PREPARATION   OF   THE   ORE. 


All  ore  assays  must  be  finely  powdered,  but  not  the 
whole  of  a  large  sample  need  be  so.  Ordinarily,  samples 
as  received  by  the  assayer  may  weigh  from  i  to  50 
pounds.  A  sample,  if  not  already  crushed,  is  broken  by 
hammer  and  block,  pestle  and  mortar,  etc.,  and,  accord- 
ing to  its  weight,  should  be  made  to  pass  a  2  or  4,  or  finer 
mesh  sieve,  and  is  then  thoroughly  mixed  on  a  smooth 
cloth  or  table.  The  sample  is  then  spread  in  a  layer  and 
divided,  by  the  back  of  a  saw  blade,  a  trowel,  or  spatula, 
into  quarters;  two  opposite  quarters  are  rejected  and 
swept  cleanly  off  the  table ;  the  other  two  are  again 
mixed,  after  further  pulverizing,  if  needful,  and  again 
divided.  When  the  sample  has  been  quartered  to  about 
10  pounds,  it  should  pass  the  4  mesh  sieve,  and  when 
quartered  to  2  to  3  pounds  should  pass  the  30  mesh  sieve ; 
when  quartered  to  about  i^  pounds  it  should  pass  the 
60  mesh  sieve;  an  assay  sample  of  about  ^  a  pound  is 
then  taken  out  by  the  use  of  riffles  or  by  quartering,  and 
is  made  to  pass  an  80  or  100  mesh  sieve.  The  sample 
always  should  be  thoroughly  mixed  before  quartering 
and  before  the  charge  is  weighed  out  for  assay.  A 
sheet  of  strong  paper  or  oil-cloth  is  suitable  to  mix  on, 
by  lifting  an  edge  and  drawing  it  forward,  not  simply 
causing  the  ore  to  slide  from  place  to  place,  but  so  as  to 
roll  the  sample  upon  itself,  and  finally  to  heap  it  in  the 
middle,  then  again  spreading  by  the  spatula,  and  so  sev- 
eral times,  after  which  it  is  again  quartered. 


AND   SILVER   ORES  47 

Dried  mill  pulp  and  tailings  samples  are  best  pre- 
pared for  assay  by  passing  them  through  a  20  mesh 
sieve,  thus  breaking  up  all  clots ;  about  ^  pound  is  then 
taken  out  by  the  use  of  riffles  or  by  quartering,  and  is 
ground  to  pass  a  60  or  80  mesh  sieve.  The  20,  60  or 
80  mesh  sieves  used  should  be  kept  specially  for  this 
work,  because  of  the  danger  of  introducing  coarse  gold 
into  the  tailings  sample. 

The  assayer  should  remember  that  the  amount  of  work 
to  put  on  any  sample  depends  upon  the  time  allowed  and 
the  facilities  for  crushing,  grinding,  etc.,  also  upon  the 
sample  itself,  whether  values  are  supposed  to  be  dis- 
tributed uniformly  or  not,  and  the  care  with  which  it 
was  originally  taken. 

For  mixing  a  sample  after  it  has  reached  a  certain 
fineness,  no  implement  is  equal  to  the  finger.  Heap  the 
ore,  then  with  the  finger  beginning  at  the  center  of  the 
heap,  trace  a  spiral  to  the  edge  and  back  to  the  center. 
Again  heap  by  lifting  the  edges  of  the  sheet,  and  again 
spread  by  the  finger,  and  so  on. 

Without  perfect  mixture  at  each  stage  of  division,  the 
final  sample  will  not  be  a  true  one,  and  this  final  sam- 
ple must  again  be  well  mixed,  in  order  that  each  assay 
taken  from  it  may  also  be  a  true  sample  of  the  whole. 
An  idea  of  what  is  required  may  be  got  by  adding  a 
little  flour  to  a  finely  powdered  sample  of  dark-colored 
ore,  or  charcoal  to  a  light-colored  one,  and  mixing  un- 
til the  tint  is  uniform,  without  streaks  or  spots. 

Each  time  that  a  sample  is  ground  and  sifted,  the 
whole  of  it  must  pass  the  sieve,  unless  there  are  parti- 
cles of  tough  matter,  such  as  metal,  or  silver  glance, 
etc.,  which  can  not  be  ground  to  a  powder;  in  this  case 


48  ASSAYING    GOLD 

the  tough  matter  must  be  kept,  and  no  further  division 
of  the  sample  can  be  made,  as  it  must  all  be  weighed 
and  treated  as  directed  further  on. 

Damp  ore  must  be  dried.  In  particular  cases  all 
samples,  whether  they  appear  damp  or  not,  should  be 
dried  after  being  powdered,  then  allowed  to  cool  be- 
fore weighing  the  assay.  Many  substances,  such  as 
clay,  for  instance,  lose  more  and  more  water  at  succes- 
sive degrees  of  heat  up  to  whiteness,  wherefore  there 
must  be  a  standard  temperature  for  the  drying.  That 
standard  is  the  heat  of  boiling  water;  hence  the  sample 
is  dried  on  the  water-bath. 

All  implements  used  in  powdering  and  sifting  must 
be  cleaned,  lest  the  sample  be  contaminated  by  some 
remains  of  a  former  one.  In  many  cases,  simply 
wiping  the  mortar,  etc.,  with  a  cloth  will  suffice,  but 
after  rich  ore,  the  tools  must  be  cleaned  by  grinding 
some  barren  quartz,  sand  or  glass.  Sieves  are  cleaned  by 
brushing  and  tapping. 

The  prepared  sample  is  put  in  a  sample-pan  with  a 
tag,  on  which  is  marked  the  number  of  the  sample,  and 
any  other  desired  particulars,  such  as  the  character  of 
the  ore,  as  a  guide  in  dressing,  or  the  metal  for  which 
it  is  to  be  assayed,  etc. 

The  character  of  the  ore  can,  in  general,  be  ascer- 
tained sufficiently,  and  most  conveniently,  by  an  exami- 
nation of  it  in  the  lump,  before  grinding.  When  this 
can  not  be  done,  as  in  the  case  of  samples  which  come 
already  ground,  recourse  must  be  had  to  washing  (pan- 
ning) a  little  of  the  powder  in  a  dish,  by  which  the 
character  and  proportion  of  sulphurets  may  be  judged, 
if  there  are   any.     The   color  and   general   appearance 


AND  SILVER  ORES  49 

also  furnish  indications,  or  special  tests  may  be  applied, 
some  of  which  are  given  further  on. 

Note — A  sample  which  consists  entirely  of  sandy  or  granular 
matter,  as  concentrates,  for  example,  is  difficult  to  mix,  because 
of  the  tendency  of  the  smaller  and  denser  particles  to  sift  between 
the  coarser  and  less  dense,  and  accumulate  at  the  bottom.  Such 
a  sample  may  be  slightly  moistened,  mixed,  and  quartered  as 
many  times  as  may  be  required,  or  it  may  be  ground  before  mix- 
ing and  quartering,  so  as  to  destroy  its  sandy  character,  and  the 
lesser  sample  thus  get  dried,  and,  if  necessary,  ground,  etc.  On 
the  other  hand,  a  sample  consisting  largely  of  impalpable  powder 
or  floury  matter  is  also  difficult  to  mix,  because  of  a  tendency  to 
clotting.  Such  material  can  not  be  mixed  by  dimply  heaping  and 
spreading,  but  should  be  passed,  little  by  little,  several  times 
under  the  spatula,  or  through  a  sieve,  so  as  to  rub  and  mash  the 
clots.  A  fair  proportion  of  fine  powder  and  grit  is  best  for  the 
mixing.  In  powdering  ore,  take  but  little  at  one  time;  work  by 
alternate  or  combined  pounding  and  rubbing,  and  sift  often, 
tapping  the  sieve  while  also  shaking  it,  to  prevent  the  meshes 
becoming  clogged  by  particles  of  ore.  It  is  best  to  weigh  out  the 
assay  immediately  after  mixing  the  sample,  as  the  heavier  and 
richer  particles  will  gradually  settle  toward  the  bottom  of  the 
mass  of  powder,  especially  if  that  is  exposed  to  vibrations  from 
the  pounding  of  other  samples  in  the  vicinity,  or  from  people 
walking  on  the  floor.  Hence,  if  a  sample  stands  long  before  the 
assay  is  weighed  out,  it  should  be  mixed  before  the  weighing  is 
done.  This  is  particularly  important  in  the  case  of  samples  which 
are  rich  in  gold. 


50  ASSAYING    GOLD 


WEIGHING  THE   CHARGE. 


Sit  or  stand  squarely  in  front  of  the  scales,  and  see 
that  they  balance  with  the  capsules  on  them.  In  spe- 
cial "pulp  scales"  the  pans  themselves  seem  intended  to 
be  used  as  capsules,  being  unattached  and  furnished 
with  handles  ;  nevertheless,  I  prefer  to  weigh  ore  or  fluxes 
in  a  small  porcelain  dish  counterpoised  by  a  piece  of  lead. 

Put  the  weights  on  the  left-hand  pan. 

After  a  little  practise,  it  will  not  be  necessary  to  weigh 
the  fluxes,  etc.,  except  in  some  cases,  niter  and  flour;  in 
general,  these  things  can  all  be  measured  closely  enough. 

The  fluxes  should  be  measured  or  weighed  first,  then 
the  pulp  weighed  and  placed  on  top  for  the  mixing. 

The  assay-ton  system  of  weights  is  so  much  more  sen- 
sible and  convenient  than  either  ounces  or  grams  that  I 
have  resolved  to  adopt  it  generally  where  I  have  occasion 
to  give  definite  quantities.  Where  only  proportions  are 
required,  the  term  parts  will  be  employed.  The  sign  for 
the  assay-ton  is  A.  T.  (See  article  on  "Calculating  the 
Assay.") 


AND   SILVER  ORES  5  I 


MIXING   AND   CHARGING. 


The  crucible  assay  may  be  mixed  on  a  sheet  of  writing- 
paper  (glazed  paper  is  better),  and  poured  from  that  into 
the  pot,  care  being  taken  to  leave  nothing  behind.  A  more 
usual  way  is  to  put  first  the  fluxes,  or  some  of  them,  and 
then  the  ore,  directly  from  the  weighing-capsule  into  the 
pot  as  it  stands  in  the  rack,  or  on  a  sheet  of  paper;  the 
mixing  is  then  done  with  the  handle  of  a  teaspoon.  The 
really  best  way  is  to  mix  by  means  of  a  pestle  and  mortar. 
The  pots  must  not  be  more  than  two-thirds  full,  on  ac- 
count of  the  swelling  of  the  charge  when  heated.  An  as- 
say should  not  be  pressed  or  packed  down  in  the  pot,  for, 
if  this  be  done,  some  substance  is  liable  to  be  blown  out 
by  the  sudden  and  forcible  escape  of  gas ;  it  may,  however, 
be  tapped  or  shaken  down  with  advantage.  Pots  which 
have  been  used  for  rich  ores  should  not  be  employed  for 
poor  ones;  and  for  important  assays,  whether  rich  or 
poor,  new  ones  should  be  taken.  This  applies  also  to 
scorifiers.  The  scorification  assay  is  mixed  in  the  scori- 
fier.  To  prepare  a  scorification  assay,  first  pour  one-half 
the  requisite  amount  of  granulated  lead  into  the  scorifier, 
add  the  ore  and  mix  with  the  spatula,  then  pour  the  re- 
maining half  of  the  lead  over  the  mixed  charge,  and 
finally  add  the  borax  glass. 


52  ASSAYING    GOLD 


ASSAY  OF  LITHARGE. 


The  litharge  frequently  contains  an  appreciable  quan- 
tity of  silver,  and  it  is  desirable  that  it  should  do  so,  as 
otherwise  the  bead  from  a  very  poor  ore,  which  may, 
nevertheless,  contain  some  gold,  might  sink  into  the  cupel 
so  as  to  escape  observation.  Each  lot,  as  opened  for  use, 
must  therefore  be  sampled  and  assayed;  the  ore  assays 
made  with  it  must  be  corrected  by  the  result.  Where,  as 
in  a  mill,  the  same  quantity  of  litharge  is  constantly  used 
in  the  assays,  the  readiest  way  of  making  the  correction 
is  to  assay  that  quantity,  and  place  the  resulting  bead  on 
the  weight-pan  when  weighing  the  beads ;  but  where  vary- 
ing quantities  of  litharge  are  used,  it  is  better  to  treat  a 
larger  quantity,  note  the  weight  of  silver  got,  and  correct 
each  ore  assay  according  to  the  litharge  used  in  making  it. 

Place  in  a  crucible  this  mixture : — 

Litharge 4  A.   T.     ' 

Flour V,„  "     "  • 

Soda    I  "     " 

Cover  with  salt,  and  melt  quickly;  when  cooled,  clean 
and  cupel  the  button.  Make  two  such  assays,  and  part 
one  of  the  beads  for  gold,  which,  however,  should  not  be 
present.  (See  articles  on  "Melting  in  Crucibles,"  "Cupel- 
lation,"  and  "Parting.") 

We  must  also  determine  beforehand  the  oxidizing 
power  of  niter  and  the  reducing  power  of  charcoal  and 
argol.  This  necessity  arises  because  of  impurities  in  the 
reagents.     By  oxidizing  power  is  meant  the  amount  of 


AND   SILVER   ORES  53 

metallic  lead  that  one  g^ram  of  niter  will  oxidize,  and  by 
reducing  power,  the  amount  of  metallic  lead  that  one 
gram  of  reagent  will  reduce  from  litharge. 

To  determine  the  oxidizing  power  of  niter,  mix  the 
following  charge,  fuse  in  clay  crucible,  and  weigh  result- 
ing button : — 

Niter 3  grams 

Charcoal   i  gram 

Litharge    60  grams 

Soda  bicarb 30  grams 

In  the  absence  of  niter,  we  would  have  obtained  a  but- 
ton of  certain  weight,  say,  27  grams,  due  to  the  charcoal 
present.  The  button  obtained  will  weigh,  say,  12  grams, 
that  is,  3  grams  of  niter  have  oxidized  27 — 12=15  grams 
of  lead,  whence  i  gram  of  niter  will  oxidize  5  grams  of 
lead. 

To  determine  the  reducing  power,  make  up  charges : — 

Argol.  Charcoal. 

Argol    I  gram  Charcoal i  gram 

Litharge   30  grams  Litharge    60  grams 

Soda    15  grams  Soda    30  grams 

Salt  cover.  .  .  .  Salt  cover.  .  .  . 

The  weight  of  the  buttons  will  tell  how  much  lead  is 
reduced  by  i  gram  of  charcoal  or  argol.  The  reducing, 
power  follows. 


54  ASSAYING    GOLD 


SYSTEMS  OF  THE  CRUCIBLE  ASSAY. 


In  all  fire-assays  of  gold  and  silver  ores,  the  precious 
metals  are  collected  by  means  of  lead,  from  which  they 
are  afterwards  separated.  In  crucible-assays  the  lead  is 
produced  from  litharge  by  the  action  of  reducers.  The 
crucible-assay  is  divided  into  two  systems,  either  of 
which  may  be  applied  to  any  ore,  but  is  best  adapted  to 
certain  cases. 

In  the  assay  by  the  second  system,  or  ''nail-assay," 
litharge  is  used  than  is  requisite  for  the  production  of 
lead;  the  excess  assists  in  fluxing  the  slag.  All  base 
metals,  except  the  required  lead,  are  oxidized  and  dis- 
solved in  the  slag.  This  assay  is  quickly  made,  and  gener- 
ally gives  accurate  results;  it  requires  some  modification 
for  the  various  ores,  as  to  the  fluxes  proper  and  the  re- 
ducers or  oxidizers  by  which  the  production  of  lead  is 
controlled ;  sometimes  a  preliminary  assay  is  necessary. 

In  the  assay  by  the  second  system,  or  "nil-assay," 
litharge  is  not  used  as  a  flux  for  the  gangue,  but  simply 
as  a  source  of  lead,  and  only  so  much  of  it  as  may  be 
necessary  for  that  purpose  is  added  in  the  dressing.  Base 
metals,  except  lead  and  perhaps  antimony,  remain  in  the 
form  of  oxidized  or  sulphureted  slag,  or  of  speiss,  that  is, 
combined  with  oxygen,  sulphur,  or  arsenic,  as  the  case 
may  be.  Antimony  sometimes  passes  more  or  less  into  the 
lead,  and,  as  it  interferes  with  cupellation  by  causing  the 
cupel  to  crack,  ores  containing  much  antimony  must  not 
be  assayed  on  this  system.    Arsenic  combines  with  nickel. 


AND   SILVER   ORES  55 

cobalt,  copper,  or  iron,  forming  a  button  which  is  usually 
hard  and  brittle,  and  which  separates  readily  from  the 
true,  or  lead  button.  If  the  ore  contains  copper,  that  will 
be  reduced,  and  will  contaminate  the  button,  unless  sul- 
phur or  arsenic  be  present  to  combine  with  it.  By  keep- 
ing such  a  charge  containing  copper  and  zinc  basic,  that 
is  by  adding  but  little  silica,  a  softer  button  will  be  ob- 
tained. 

The  method  requires  a  higher  heat  and  longer  time 
than  the  other;  it  can  not  always  be  depended  upon  for 
a  true  result  as  to  the  silver;  the  gold  is  got  with  more 
certainty.  This  system  is  convenient  for  the  determina- 
tion of  gold  in  auriferous  pyrites,  also  in  cases  where 
litharge  and  crucibles  are  expensive,  and  where  great 
accuracy  is  not  requisite  as  to  the  silver,  or  where  the 
ore  is  of  low  grade,  so  that  error  is  not  of  great  impor- 
tance. 

With  some  ores,  especially  those  rich  in  lead  and  not  so 
in  copper,  the  second  system  gives  as  good  results  as  any ; 
yet,  on  the  whole,  it  must  be  conceded  to  be  an  inferior 
process  as  to  silver.  It  is  sometimes  convenient  to  make  a 
small  assay  by  the  first  system  for  the  silver  especially, 
and  a  much  larger  one  by  the  second  for  the  more  accu- 
rate determination  of  the  gold  in  an  ore. 

The  following  reactions  will  give  an  idea  of  the  chem- 
istry of  the  two  processes,  and  will  indicate  what  should 
be  added  to  the  charge  to  get  certain  desired  results : — 


56  ASSAYING    GOLD 

FIRST  SYSTEM. 
2NaHC03    +    heat    =    NaXOg    +    CO,    +    H^O 

Sodium  bi-carbonate.  Sodium  carbonate.  Carbon  dioxide.  Water. 

2Na2C03    +     SiO^     =     2Nr,0,  SiO,     +    2CO, 

Silica.  Mono-silicate  of  sodium. 

(Slag.) 

FeS^     +     sPbO     =     FeO       +       2SO2       +       S^b 

Iron  pyrites.      Litharge.  Ferrous  iron.         Sulphur  dioxide.  Lead. 

4FeS2  +  10KNO3  =  4FeO  +  5K2SO4  +  3SO2  +  loN 

Nitre.  Potassium  sulphate.  Nitrogen. 

2FeO       +       SiO^       =       2FeO,  SiO^ 

Mono-silicate  of  iron. 
(Slag.) 

2PbO        -I-        C        =        2Pb        +        CO. 
Carbon. 

2PbO      +      SiOs      =:      2PbO,    SiO^ 

Mono-silicate  of  lead. 
(Slag.) 

Borax  assists  in  fluxing  by  forming  fusible  complex  bo- 
rates with  oxides  of  the  base  metals. 


SECOND  SYSTEM. 
2FeS--  +  2PbO  =  Pb  +  2FeS,  PbS  +  SO3 

Double  sulphide 
of  iron  and  lead. 

Na^COa  +  FeS.  =  Na.S,  FeS  +  CO^ 

Double  sulphide  of 
iron  and  sodium. 

FeS^  +  Fe  -  2FeS 

Iron.  Ferrous  sulphide. 

2FeS,  PbS  +  Fe  =  sFeS  +  Pb 


AND   SILVER   ORES  57 


PRELIMINARY  ASSAY. 


In  the  crucible-assay  by  the  first  system,  ores  which 
contain  a  great  proportion  of  sulphtirets  would  reduce  too 
much  lead  from  the  litharge;  others  containing  less 
would  reduce  the  right  quantity ;  others,  again,  too  little 
or  none,  and  an  ore  containing  a  large  proportion  of  the 
higher  oxides  of  lead,  copper,  iron,  manganese,  or  chro- 
mium would  prevent  the  reduction  of  lead  by  a  reducer 
unless  the  latter  were  present  in  sufficient  quantity  to 
counteract  their  oxidizing  effect  and  leave  a  surplus  for 
reduction.  An  excess  of  reducing  power  in  the  ore  may 
be  counteracted  by  means  of  niter/^ 

There  are  two  limits  to  the  size  of  the  button ;  it  must 
be  large  enough,  enough  litharge  having  been  reduced 
through  the  mass  to  collect  all  the  precious  metal,  and  at 
the  same  time  there  should  not  be  a  useless  excess  of  the 
lead,  which  would  occasion  loss  of  silver  in  the  subsequent 
cupellation.  Fifteen  to  twenty  grams  is  the  best  size  for 
from  one-third  to  four  A.  T.  of  ore,  and  is  the  best  size 
for  cupelling. 

An  experienced  workman  can  generally  judge  what  to 
do,  for,  although  the  weight  of  lead  got  ought  not  to  be 
much  less  than  ninety  per  cent  of  that  of  the  ore  used,  yet 
there  is  a  considerable  margin  for  variation  on  the  other 
side.     The  writer   rarely  makes   a   preliminar}-,   but   in 


*I  have  used  manganese  dioxide  instead  of  niter  with  good 
results.  An  assay  so  made  may  require  an  extra  dose  of  borax 
or  glass  if  the  gangue  is  basic,  as  lime,  etc. 


58  ASSAYING    GOLD 

doubtful  cases  dresses  the  assay  according  to  his  judg- 
ment, with  more  or  less  of  the  various  fluxes  according  to 
the  nature  of  the  gangue,  and  with  an  addition  of  niter 
or  an  extra  dose  of  flour,  as  the  case  may  be;  this  is, 
however,  difficult  when  the  ore  consists  of  a  mixture  of 
oxidizer  and  reducer,  as  copper  glance  and  iron  red  oxide, 
etc.  In  case  a  preliminary  is  required,  it  may  be  made  as 
follows:  Consider  whether  the  ore  is  likely  to  be  a  re- 
ducer (that  is,  it  contains  a  considerable  proportion  of 
sulphurets),  or  an  oxidizer  (that  is,  carries  much  red  ox- 
ide of  iron,  black  oxide  of  manganese,  black  oxide  or 
blue  or  green  carbonate  of  copper,  red  lead  or  a  chro- 
mate).  If  it  is  a  reducer,  take  one-tenth  A.  T.  of  ore  and 
5  A.  T.  of  litharge;  mix  in  a  crucible,  cover  with  salt, 
and  melt  with  care,  that  no  coal  falls  in;  the  weight  of 
the  resulting  button  above  o.i  A.  T.  multiplied  by  2.5  is 
the  weight  of  niter  required  in  an  assay  of  i  A.  T.  of 
the  ore ;   for  a  half-ton  assay  take  half  that  quantity,  etc. 

If  the  button  weighs  much  less  than  o.i  A.  T.,  the  def- 
icit multiplied  by  ten  times  the  fraction  of  an  A.  T.,  rep- 
resenting the  quantity  of  flour  which  would  bring  down 
I  A.  T.  of  lead  from  the  litharge,  is  the  weight  of  flour 
to  be  used  in  an  assay  of  i  A.  T.  of  the  ore. 

The  proportion  of  lead  which  the  flour  will  bring  down 
depends  on  the  quality  of  the  flour  and  the  purity  of  the 
litharge;  it  varies  from  12  to  16  times  the  weight  of  the 
flour,  and  may  be  readily  found  by  melting  3  A.  T.  of  the 
litharge,  mixed  with  some  soda  and  0.1  A.  T.  of  the 
flour,  under  a  salt  cover,  then,  after  cooling,  weighing 
the  resulting  button. 

If  the  ore  brings  down  no  lead,  it  is  not  a  reducer ;  if 
it  brings  down  from  90  to  100  per  cent  of  its  own  weight. 


AND   SILVER   ORES  59 

it  may  be  dressed  for  the  assay  proper  without  either 
niter  or  flour. 

If  the  ore  is  supposed  to  be  an  oxidizer,  melt  one-tenth 
A.  T.  of  it  with  about  2  A.  T.  of  Htharge,  some  borax, 
and  so  much  flour  as,  according  to  the  above  described 
experiment,  should  bring  down  a  certain  quantity  of 
lead.  Now,  if  you  get  that  quantity,  the  ore  is  not  an 
oxidizer;  if  you  get  less,  it  is,  and  the  deficit  is  the 
measure  of  its  oxidizing  power;  then  the  difference  be- 
tween the  weight  of  lead  got  and  that  which  should  have 
been  got  if  no  ore  had  been  present  multiplied  by  ten 
times  the  number  of  A.  T.  of  flour  which  was  found  to 
be  necessary  to  bring  down  i  A.  T.  of  lead  from  the 
litharge,  is  the  weight  of  extra  flour  to  be  used  in  an 
assay  of  i  A.  T.  of  the  ore;  half  as  much  for  one-half 
A.  T.,  etc.  Suppose  it  had  been  found  that  0.08  A.  T. 
of  flour  would  bring  down  i  A.  T.  of  lead,  and  we  take 
0.1  A.  T.  of  ore  and  0.02  A.  T.  of  flour  with  2  A.  T.  of 
litharge,  etc. ;  suppose  we  get  o.ii  A.  T.  of  lead,  that  is, 
0.14  A.  T.  less  than  the  quantity  called  for  by  the  flour; 
then  0.14,  multiplied  by  0.8,  gives  0.112  A.  T.  of 
extra  flour  required  for  i  A.  T.  of  the  ore.  Or, 
we  might  weigh  a  full  assay,  a  ton  or  half  ton,  of  the 
ore,  and  dress  it  regularly  as  hereafter  directed,  adding 
so  much  extra  flour  as  we  judge  to  be  required ;  then,  if 
a  suitable  button  be  got,  it  may  be  cupelled  for  the  pre- 
cious metal ;  otherwise,  another  assay  may  be  made  with 
the  required  correction. 

If  fractional  A.  T.  weights  are  not  at  hand,  the  work 
can  be  done  in  grams ;  but  the  A.  T.  weights  can  easily  be 
made  of  lead  or  other  metal,  down  to  o.oi  or  lower,  or  the 
parts  of  the  ton  may  be  reduced  to  grams  in  the  form  of 


6o  ASSAYING    GOLD 

a  table  to  be  kept  at  hand.  The  main  point  is  to  remem- 
ber that  I  part  of  niter  is  equal  to  the  oxidation  of  from 
y/2  to  4  parts  of  lead,  according  to  its  purity,  or  an  equiv- 
alent of  sulphurets,  and  i  part  of  flour  reduces  from  12 
to  16  parts  of  lead  from  litharge,  or,  which  is  the  same 
thing,  from  0.06  to  0.08  ton  of  flour  brings  down  i  ton  of 
lead,  the  exact  proportion  to  be  found  by  experiment. 


AND   SILVER   ORES  6l 


DRESSING  THE  CRUCIBLE  ASSAYS. 


FIRST  SYSTEM. 

From  what  has  been  said  of  the  properties  of  the  dif- 
ferent fluxes,  it  will  be  seen  that  the  proportions  in  which 
they  are  used  may  be  varied  to  suit  the  character  of  the 
ore.  Mitchell  gives  one  general  formula  for  all  ores  of 
gold  and  silver,  as  follows :  — 

Ore    I  part 

Litharge    5     '' 

Soda I     "      • 

Borax  glass i     " 

Salt  to  cover,*  another  part  of  borax  over  that,  and 
reducer  (flour)  or  oxidizer  (niter),  if  needed,  in  ac- 
cordance with  the  indications  given  by  a  preliminary. 

This  formula  is  of  extensive  applicability,  admitting 
the  variation  in  respect  of  flour  or  niter,  and  it  is  well  to 
keep  the  proportions  in  mind;  but  the  same  objections 
apply  in  a  less  degree  to  this  as  to  the  exclusive  use  of 
litharge  as  a  flux,  with  flour  or  niter  to  suit.  These  ob- 
jections are  the  expense  for  litharge,  which  is  compara- 
tively costly,  and  the  rapid  destruction  of  crucibles,  which 

*Some  assayers  omit  the  salt,  thinking  that  it  only  serves  for 
washing  the  sides  of  the  pot,  and  may  be  dispensed  with  for 
economy's  sake.  It  is  certainly  not  indispensable,  but  it  assists 
greatly  in  preventing  an  overflow,  by  its  easy  fusibility  imparting 
a  quasi-liquid  condition  to  the  assay  at  an  early  stage,  thus  facil- 
itating the  escape  of  expanding  gas,  especially  if  mixed  instead 
of  being  put  on  the  top. 


62  ASSAYING    GOLD 

is  an  important  item  in  districts  remote  from  a  city,  and 
where,  as  in  mills,  a  great  many  assays  must  be  made 
daily.  In  a  majority  of  cases,  3  parts  of  litharge  suffice ; 
in  many  2  are  enough,  and  in  some  i^,  but,  when  the 
litharge  is  thus  spared,  it  may  be  necesary  to  alter  the  pro- 
portions of  the  other  fluxes,  which  the  student  will  be 
enabled  to  do  by  a  careful  study  of  their  action.  It  should 
be  remembered  that  soda  is  a  flux  for  quartz;  borax  for 
earths  and  metal  oxides  generally,  as  is  also  the  glass 
formed  by  soda  with  quartz ;  litharge  for  all. 

As  to  making  a  preliminary  every  time,  it  is  out  of 
the  question.  The  workman  must,  in  many  cases,  rely  on 
his  judgment,  or  have  recourse  to  the  second  system,  or 
''nail-assay."  For  ordinary  ores,  containing  little  or  no 
sulphuret,  some  quartz,  clay,  lime,  iron  oxide,  etc.,  this 
formula  will  in  general  suffice: — 


Ore I  part 

Litharge 2     " 

Soda I     " 

*Dried  borax  .  .  .  >^     " 

Flour 1-12     " 

Salt  to  cover. 


Fuse  quickly,  keep  in 
furnace  5  to  10  minutes 
after  subsidence. 


and  I  part  borax  on  top. 

If  the  ore  is  nearly  all  quartz,  the  borax  may  be  re- 
duced and  the  soda  increased;  or,  as  much  soda  may 
cause  the  assay  to  boil  up  too  much,  the  litharge  may  be 
increased  instead.  On  the  other  hand,  if  earthy  matter 
or  metal  oxide  predominate,  the  soda  may  be  reduced 

*Or  rather  less  of  dry  soda  ash,  and  borax  glass, 


AND   SILVER  ORES  63 

one-half  or  more,  and  the  borax  increased  even  to  doub- 
Hng,  or,  in  place  of  so  much  borax,  some  glass  may  bt 
added,  in  which  case  it  may  be  better  to  retain  all  the 
soda. 

If  the  button  got  is  too  large,  it  may  be  scorified  to  a 
suitable  size,  but  if  the  assay  is  very  important,  it  will  be 
better  to  make  another  with  less  flour  in  the  ratio  indi- 
cated by  experiment.  (See  article  on  "Preliminary  As- 
say.") If  the  button  is  too  small,  repeat  with  more  flour 
in  the  same  ratio  to  the  deficit.  Thus  the  assay  answers 
all  the  purposes  of  a  preliminary,  while,  if  the  result  is 
satisfactory,  time  is  saved.* 

If  the  ore  contains  sulphurets,  less  flour  must  be  used 
or  none  may  be  needed ;  or,  again,  niter  may  be  employed. 

If  silver  only  is  to  be  determined,  or,  rather,  precious 
metal,  not  parted,  and  a  delicate  balance  is  accessible,  a 
quarter  A.  T.,  or  even  a  smaller  quantity  of  the  ore,  may 
be  used,  and,  being  melted  with  a  plenty  of  litharge,  ac- 
cording to  the  nature  of  the  ore,  the  button  may  not  be 
too  large  for  immediate  cupellation ;  this  is  an  excellent 
method  for  argentiferous  galena,  silver  glance,  and  such 
sulphureted  ores  generally  as  do  not  throw  down  an  in- 
ordinate proportion  of  lead. 

*This  will  not  answer  with  an  ore  containing  a  large  proportion 
of  sulphurets  other  than  galena  or  silver  glance,  because  the  pro- 
portion of  litharge  used  will  not  suffice  for  the  complete  decom- 
position of  the  sulphurets.     A  preliminary  is  then  proper. 


64 


ASSAYING    GOLD 


The  proportion  of  some  siilphurets  which  will  bring 
down  a  suitable  quantity  of  lead  are : — 

For  silver. 

Iron  pyrites 1 1  per  cent 

Copper  pyrites 12 

Zinc-blende 13 

Sulphuret  of  antimony.  ..  16 

Gray  copper 15 

Galena 34 

Sulphuret  of  copper 23 

Sulphuret  of  manganese.  .  13 
Any  excess  may  be  met  by  means  of  niter. 

It  is  important  that  the  slag  should  be  thoroughly  oxi- 
dized, retaining  no  vestige  of  a  sulphuret.  The  different 
kinds  of  sulphurets  require  different  proportions  of  lith- 
arge to  effect  complete  oxidation,  as  follows — only  a  small 
part  of  the  litharge  is  reduced  to  lead. : — 


For  gold. 
\2.y2  per  cent 

i6>^ 
13 


25 


I   gram  of  charcoal  will  reduce 
I        "       "  flour           "           " 
I        "       "  argol           "            " 
I        "       "  Fe   pyrites   will   redu 
I       "       "  Cu       " 
I       "       "  Zn  sulphide    "           " 
I       "       "  Sb 
I       "       "  Pb 

about  30  grams  ol 
15       )^'       '* 

■  lead. 

ce  about   lo  gr.,   i 

::    ^•; 

7  " 
3  " 

equire 

50 
30 

25 

25 

5 

gr.   PbO   to   ox 

The  following  table  will  also  prove 

useful : 



%  of  pyrite 

amount  of  ore 

lead  reduced 

2 

1  A.  T. 

5 

grams. 

5 

u 

12 

" 

10 

" 

25 

« 

15 

V2  K.T 

19 

" 

20 

u 

25 

« 

30 

" 

.37 

« 

40 

« 

50 

-'» 

50 

(( 

63 

« 

70 

u 

88 

« 

100 

u 

125 

« 

AND   SILVER   ORES  65 

The  excess  of  lead  reduced  by  ores  of  the  type  studied 
is  oxidized  by  niter,  i  gram  of  niter  oxidizing  4  grams 
of  lead,  or  we  may  express  its  oxidizing  power  in  terms 
of  the  reducing-agent,  as  follows : — 

2  Yz  grams  of  KNO3  will  oxidize  1  gram  of  pyrite. 

1  H      "         "        "        "            "  1  "        "  stibnite. 

V3      "         "        "        "            "  1  "        "  galena. 

Ya      "         "        "        "           "  1  "        "  lead. 

Some  workmen  use  nails  with  these  ores  in  this  system 
of  assay.  No  doubt  the  iron  will  prevent  the  formation 
of  any  lead  matte,  but  as  iron  is  a  reducer,  it  is  liable  to 
throw  down  too  much  lead;  it  also  reduces  copper  if 
present. 

The  simultaneous  use  of  niter  and  flour  or  charcoal 
in  certain  assays  is  a  relic  of  ancient  practise,  based  on 
somewhat  misty  theories  of  reaction  at  different  tem- 
peratures, and  is  retained  by  some  who  might  be  ex- 
pected to  know  better,  but  who  seem  to  occasionally  pre- 
fer the  rule  of  thumb  to  the  rule  of  reason.  The  student 
may  rest  assured  that,  in  an  assay  for  gold  and  silver, 
when  flour  is  needed  niter  is  not. 

Some,  even  chemists,  put  niter  into  an  oxidized  coppery 
ore,  with  the  insane  idea  of  "burning  out  the  copper." 
Every  chemist  ought  to  know  that,  in  the  first  place,  the 
copper  is  never  burned  out,  but  remains  in  the  slag ;  and, 
secondly,  that  it  must  be  slagged  in  the  form  of  red  sub- 
oxide, not  as  a  green  glass.  The  black  oxide  or  the  car- 
bonate is  an  oxidizer  in  an  assay,  and,  so  far  from  requir- 
ing niter,  calls  for  an  extra  quantity  of  reducer  to  bring 
it  to  the  required  condition.  The  red  oxide  of  copper  in 
an  ore  is  already  in  the  proper  condition,  and  for  finely 
divided  metallic  copper  such  as  can  pass  the  assay-sieve. 


66  ASSAYING    GOLD 

litharge  in  liberal  quantity  is  the  safe  and  appropriate 
oxidizer,  which  will  convert  it  into  the  red  oxide  with 
no  danger  of  forming  a  green  slag. 

That  a  coppery  button  is  sometimes  got  from  an  assay 
by  this  system  is  due  to  the  fact  that  lead  can  reduce  the 
copper  oxide  to  metal,  unless  a  sufficient  quantity  of  lith- 
arge is  present,  when  the  copper  remains  in  the  slag  as 
red  oxide ;  hence,  if  the  button  is  much  contaminated  by 
copper,  the  need  of  more  litharge  in  the  assay  is  indi- 
cated.* 

One  way  in  which  to  get  rid  of  all  difficulty  as  to  the 
proportion  of  niter  is  to  use  so  much  as  to  completely 
oxidize  every  constituent  of  the  ore,  decomposing  any 
excess  by  heat,  and  throwing  down  a  button  by  means  of 
charcoal. 

A  neat  way  in  which  to  make  the  assay  of  gold  and 
silver  ores  containing  much  copper  is  to  treat  the 
weighed  assay  with  hot  nitric  acid  until  the  copper  is  dis- 
solved ;  filter  and  wash.  To  the  filtrate  and  washings  add 
hydrochloric  acid  or  a  dilute  solution  of  common  salt  as 
long  as  a  precipitate  is  produced.  Collect  the  precipitate 
on  a  separate  filter,  and  wash  it.  The  whole  of  the  silver 
will  be  on  the  filters,  that  portion  which  existed  in  the  ore 
as  chloride,  iodide,  or  bromide  on  the  first,  the  remainder 
on  the  second,  also  converted  into  chloride.  It  will  not 
answer  to  add  salt  or  hydrochloric  acid  during  the  treat- 
ment of  the  ore,  because  the  proper  quantity  can  not  be 
known,  and  an  excess  would  cause  gold  to  be  dissolved 
and  lost,  if  present. 

Dry  the  two  filters  with  their  contents ;  dress  the  residue 

♦Basic  charge. 


AND   SILVER  ORES  67 

from  the  ore  with  Htharge,  borax,  and  soda  as  a  crucible 
assay  by  the  first  system,  proportioning  the  fluxes  to  the 
quantity  of  substance.  If  there  should  be  much  separated 
sulphur,  some  niter  may  be  needed.  Put  the  whole  into 
a  crucible.  Dress  the  other  filter  with  a  small  quantity  of 
litharge  and  soda,  and  put  it  als6  in  the  pot.  In  adjust- 
ing the  flour  or  niter,  it  must  be  remembered  that  the 
filters  will  act  as  reducers,  unless  they  have  been  burned 
to  ashes  in  the  drying,  which  may  be  done  on  a  roasting- 
dish  in  the  muffle  very  speedily.  Cover  with  salt,  and  on 
that  put  some  borax.  Melt  and  proceed  as  usual.  Assays 
made  in  this  way  work  very  cleanly  and  quickly,  as  there 
is  not  much  besides  quartz  and  silver  chloride  to  melt. 
The  chief  advantage  is  that  a  large  quantity  of  ore  can  be 
taken,  it  being  greatly  reduced  by  the  acid  treatment. 

In  using  niter  to  counteract  the  excess  of  reducing- 
matter,  it  may  happen  that,  although  the  niter  has  been 
adjusted  in  accordance  with  the  indications  of  a  prelimi- 
nary, little  or  no  lead  is  got. 

The  natural  supposition  then  is  that  too  much  niter 
has  been  used;  this  is  an  error.  On  repeating  the  assay 
with  a  smaller  proportion  of  niter,  the  result  is  no  better 
than  before,  while,  if  it  be  made  with  a  slightly  larger 
proportion,  the  apparently  paradoxical  result  is  that  more 
lead  is  got,  though  not  the  full  quantity  called  for  origin- 
ally; this  has  puzzled  many  assayers,  and  is  understood 
by  very  few ;  it  has  led  some  to  the  practise  of  using  nails 
in  a  nitered  assay,  at  the  risk  of  getting  too  much  lead, 
and  of  forming  oxysulphides  in  the  slag,  which  is  stated 
on  good  authority  to  be  dangerous.  It  is  true  that  I  have 
suggested  the  use  of  a  small  quantity  of  niter  in  the  assay 
of  such  ore  by  the  second  system,  but  the  litharge  used 


68  ASSAYING    GOLD 

in  that  is  not  in  excess  of  the  quantity  of  lead  required, 
and  too  much  lead  can  not  be  got,  while  the  little  niter 
intended  to  be  used,  if  any,  is  only  enough  to  burn  a  por- 
tion of  the  excess  of  sulphur  which  exists  in  iron  pyrites 
beyond  what  is  required  to  retain  the  iron  in  the  form 
of  a  sulphide  containing  the  minimum  proportion  of  sul- 
phur, to  which  condition  it  must  be  reduced  in  order  to 
insure  the  extraction  of  the  precious  metal. 

The  difficulty  spoken  of  is  caused,  not  by  an  excess  of 
niter,  but  by  a  deficiency  of  litharge.  It  has  already  been 
stated  that  iron  pyrites  require  50  parts  of  litharge  to 
effect  complete  oxidation,  reducing  only  8  or  11  parts  of 
lead,  that  is,  very  much  less  than  the  entire  quantity  of 
lead  in  the  litharge ;  the  reason  of  this  is  that  when  lith- 
arge is  combined  with  a  certain  proportion  of  iron  oxide 
it  loses  the  power  of  oxidizing  sulphurets.  When  the  ore 
consists  almost  entirely  of  iron  pyrites,  which  in  this  class 
of  assay  must  be  completely  oxidized,  a  large  quantity 
of  iron  oxide  is  formed;  this  engages  a  corresponding 
quantity  of  the  litharge,  rendering  that  powerless  to  de- 
compose sulphurets  of  any  kind,  even  galena.  Now  sup- 
pose we  use  niter  enough  to  oxidize  about  90  per  cent  of 
the  pyrites,  the  remaining  10  per  cent  (the  ore  being  prac- 
tically all  pyrites)  requires  50  times  as  much  litharge,  that 
is,  5  times  the  weight  of  the  entire  assay.  Besides  this, 
a  quantity  of  litharge  is  required  to  combine  with  the  iron 
oxide  produced  by  the  action  of  the  niter  on  90  per  cent 
of  the  pyrites,  otherwise  the  10  per  cent  of  pyrites  can 
not  be  oxidized,  because  the  litharge  intended  to  act  on 
them  is  otherwise  employed.  The  result  is  that  little  or 
no  lead  is  got,  since  little  or  no  litharge  is  reduced;  it  re- 
mains in  the  slag,  partly  combined  with  iron  oxide  and 


AND  SILVER  ORES  6^ 

partly  with  a  portion  of  the  sulphur  of  the  lo  per  cent 
of  the  pyrites. 

It  can  now  be  seen  why  more  niter  may  cause  more 
kad  to  be  reduced  by  leaving  less  than  lo  per  cent  of  the 
pyrites  to  be  acted  on  by  the  litharge,  for  although  more 
iron  oxide  is  produced,  it  does  not  consume  litharge  in 
the  same  proportion  as  is  required  by  the  pyrites ;  the  re- 
maining quantity  of  the  latter  is  then  completely  oxidized, 
and  an  equivalent  quantity  of  lead  is  thrown  down.  If 
a  sufficient  quantity  of  litharge  had  been  used,  instead  of 
more  niter,  the  lo  per  cent  or  so  of  pyrites  would  have 
been  oxidized,  and  the  calculated  quantity  of  lead  got. 
The  indications  of  a  well-made  preliminary  may  always  be 
relied  on  if  enough  litharge  is  used  in  the  assay  proper. 
From  the  foregoing  it  appears  that  the  assay  of  this 
class  of  ore  by  the  first  system,  using  a  graduated  quan- 
tity of  niter,  must  require  more  than  the  5  parts  of  lith- 
arge prescribed  for  all  ores  by  Mitchell. 

For  well-concentrated  pyrites,  the  following  dressing 
is  suitable: — 

Ore I        part 

Niter    1.7 

Litharge    8-10 

Soda 1-2 

Borax i 

Glass ^-i 

and  salt  to  cover. 

The  litharge  may  be  lessened  as  the  proportion  of  soda 
and  glass  is  increased. 

When,  on  pouring  an  assay  fluxed  according  to  the  first 
system,  it  is  found  that,  although  the  slag  is  sufficiently 
liquid  (or  otherwise)  the  lead  is  scattered,  beads  of  it 


70  ASSAYING    GOLD 

clinging  to  the  walls  of  the  crucible,  it  indicates  either 
that  the  assay  has  not  been  sufficiently  heated,  or  that  not 
enough  borax,  silica,  or  litharge  has  been  employed.  The 
assay  may  be  at  once  returned  to  the  crucible  from  the 
mold,  provided  that  no  part  of  the  slag  has  been  lost  by 
overflowing,  and  remelted  with  the  necessary  correction; 
it  can  then  be  cleanly  poured. 

An  ore  which  contains  a  very  large  proportion  of  iron 
oxide,  and  not  any,  or  very  little,  sulphuret,  works  better 
with  only  about  i;^  part  of  litharge,  but  an  addition  of 
glass  is  requisite.  I  prefer  glass  to  the  precipitated  silica 
recommended  by  some,  because  an  excess  is  less  liable 
to  make  a  pasty  slag,  the  glass  itself  being  fusible. 

SECOND  SYSTEM. 

The  following  is  a  general  formula  for  assays  by  the 
second  system : — 


Ore    I       part,  ' 

Litharge   i>^      " 

Soda    3         " 

Melt     and     leave     in 

Borax    ^      " 

Flour i-io     " 

>           strong     fire     about     20 
minutes   after   fusion. 

Iron .  .  I  to  3  nails 

Salt  to  cover. 

The  purpose  of  the  nails  is  to  free  the  lead  from  sulphur 
and  arsenic.  Twelvepenny  nails  are  suitable ;  if  too  long, 
cut  them ;  thick  wire  is  equally  good. 

When  the  ore  contains  a  great  quantity  of  pyrites,  a 
little  niter  may  be  used ;  it  assists  by  removing  a  part  of 
the  excess  of  sulphur  which  pyrites  contain;  the  flour 
should  then  be  omitted.    If  the  button  is  too  small  (sup- 


AND   SILVER   ORES  7 1 

posing  the  assay  to  have  contained  the  proper  quantity  of 
litharge  or  lead  in  some  form),  or  if  it  contains  sulphur 
or  arsenic,  the  nails  have  been  removed  too  soon  (see 
article  on  "Melting"),  or  too  little  soda  has  been  used; 
if  it  is  coppery,  not  enough  sulphur  has  been  present,  and 
in  such  case,  that  is,  when  the  ore  is  more  or  less  oxidized 
and  at  the  same  time  contains  copper,  a  little  sulphur  may 
be  added  in  the  dressing,  but  the  silver  will  not  all  be  ob- 
tained ;  it  may  fall  short  by  as  much  as  ten  per  cent,  and 
the  more  sodium  sulphide  is  formed  the  less  silver  is  got, 
so  that  if  sulphur  is  added,  it  should  be  no  more  than  is 
requisite  to  keep  copper  out  of  the  button. 

In  no  case  should  there  be  any  distinct  matte  above 
the  button,  but  all  the  metal  sulphides  should  be  blended 
in  a  black  slag. 


72  ASSAYING    GOLD 


EXAMPLES  OF  DRESSING. 


The  borax  was  only  air  dried. 

Eureka  Mine,  Arizona. 


The  flour  was  varied 


Ore     10  grams  "l 

(^   J  K      \  according    as    the    ore 

'  contained   more   or   less 

oxidizing  matter. 


Borax 15 

Salt  to  cover. 


This  ore  consisted  of  oxide  and  carbonate  of  iron, 
black  oxide  of  manganese,  carbonate  of  lime,  clay, 
silica,  and  a  sprinkling  of  molybdate  of  lead  and  horn 
silver.  The  assay  fused  very  quietly,  and  gave  ex- 
cellent buttons  and  a  glassy  slag. 

Silver  King  Mine,  Arizona. 

Ore 10  grams 

Litharge 40       " 

.  Soda 7       " 

Borax 5 

Flour   ^       " 

Salt  to  cover. 

The  ore  contained  quartz,  calcite,  porphyry,  blende, 
galena,  a  little  pyrites,  heavy  spar,  and  much  native 
silver. 


AND  SILVER  ORES  73 

Material  from  the  concentrators : — 

Tailings    lOO  grams 

Litharge    125       " 

Soda    125       " 

Borax    10       " 

Flour    I       " 

Salt  to  cover. 

Concentrations    10  grams 

Litharge 50       " 

Soda    10       " 

Borax 10       " 

Niter    i 

Scorification  worked  well  with  8  parts  lead  and  some 
borax  glass. 

The  same  ore  by  the  second  system :  — 
Ore 20  grams    \ 

T  -Vl ^        ^^  "  I 


c^    -i  u         I  Kept  in  furnace  20 

^  ,,  1"      minutes     after     subsi- 

Borax   10  , 

,,  dence. 

Flour   ....   2  I 


\ 

I 
Nails    ....   3       '■'        J 


Concentrations    10  grams 

Litharge    20      " 

Soda 30      " 

Borax 5       " 

Flour. 

Nails    3       " 

The  ores  of  the  Blind  Spring  District,  consisting  of 
partzite,  mixed  with  quartz,  oxide  of  iron,  and  manga- 
nese, gave  good  results  with  three  or  four  parts  of  lith- 
arge, flour,  and  either  soda  or  borax,  or  both.    With  soda. 


V 


74  ASSAYING    GOLD 

and  not  borax,  the  slag  was  as  liquid  as  could  be  desired, 
but  when  cold  was  granular.  With  borax  the  slag  was 
glassy.  The  results  were  equal.  At  San  Andres,  in  Hon- 
duras, being  desirous  of  economizing  both  the  pots  and 
the  litharge,  and  the  ore  being  nearly  pure  quartz,  with  a 
little  clay  and  iron  oxide,  I  used  the  following  dressing : — 

Ore    I  part 

Litharge     2     " 

Soda    I     " 

Borax    glass    .  .  .  .  i     " 

Charcoal     1-30" 

A  salt  cover  was  dispensed  with.  Later,  having  to 
economize  still  more,  I  further  reduced  the  borax  and 
litharge  while  slightly  increasing  the  soda;  the  slag  then 
contained  scarcely  any  lead,  being  simply  a  glass  of  soda 
and  quartz  with  some  borax  and  the  iron  oxide  of  the 
ore,  through  which,  when  cold,  the  button  was  plainly 
visible;  this  was  a  gold  ore.  Some  check  assays  made 
with  plenty  of  litharge  gave  no  better  results.  At  Cura- 
ren,  also  in  Honduras,  the  ore  was  mainly  of  silver,  the 
gangue  quartz,  with  a  very  small  proportion  of  iron  py- 
rites.   In  this  case  I  got  along  very  well  with : — 

Ore    I  part 

Litharge    i     " 

Soda    2     " 

Borax a  trifle 

and  flour  enough  to  reduce  all  of  the  litharge,  so  that  the 
slag  contained  no  lead.  For  the  oxidation  of  the  very 
small  portion  of  sulphurets  present  I  trusted  to  the  action 
of  the  soda,  the  litharge  being  reduced  by  the  flour.  The 
slag  was  a  perfectly  clear  glass. 


AND   SILVER   ORES  75 


THE  MELTING  IN   CRUCIBLES. 


The  furnace  should  have  a  damper.  The  fuel  should 
be  in  pieces  not  larger  than  an  egg,  nor  much  smaller  than 
a  walnut,  that  is,  for  the  general  purpose  of  melting  in 
crucibles.  The  finer  portion  can  be  used  advantageously 
when  working  the  muffle,  as  less  draft  is  then  required, 
and  by  placing  fine  fuel  on  the  top  of  the  fire  the  muffle  is 
more  evenly  heated  in  all  parts.  The  fine  fuel  is  also 
useful  when  the  fire  burns  unequally,  as  by  it  the"  draft 
can  be  checked  in  any  particular  part.  When  the  muffle- 
furnace  is  used  for  the  crucibles  also,  it  is  sometimes  desi- 
rable to  use  a  rather  short  muffle,  not  extending  entirely 
across  the  furnace,  the  rear  end  being  supported  by  a 
piece  of  fire-brick  or  an  old  crucible ;  this  allows  of  cruci- 
bles being  placed  at  the  rear,  as  well  as  along  the  sides  of 
the  muffle.  If  the  muffle  is  removed  in  order  to  save  it 
and  give  more  room  for  crucibles,  it  should  not  be  re- 
placed without  first  removing  a  portion  of  the  glowing 
coals  and  placing  a  bed  of  cold  fuel  on  the  remainder; 
the  removed  hot  coals  can  be  put  above  the  muffle,  to- 
gether with  fresh  fuel ;  in  this  way  a  fracture  of  the  muffle 
by  intense  and  sudden  heat  is  prevented.  When  the  muf- 
fle is  removed  for  the  melting,  the  opening  in  the  front 
part  of  the  furnace  which  it  occupied  may  be  closed  by 
a  plug  of  fire-clay,  which  may  be  conveniently  made  by 
cutting  off  about  two  inches  of  the  closed  end  of  an  old 
muffle ;  this  is  easily  done  by  means  of  a  coarse  file.  When 


V 


^6  ASSAYING    GOLD 

many  meltings  are  to  be  made,  it  is  always  better  to  have 
the  muffle  out. 

To  place  the  crucible  in  the  fire,  make  a  nest  for  it 
among  the  glowing  coals,  seize  it  with  the  bent  tongs  in 
one  hand,  place  and  sustain  it,  while,  with  the  cupel-tongs 
in  the  other  hand,  the  coals  are  packed  around  it,  If  the 
fuel  is  charcoal,  a  cover  must  be  used  to  prevent  bits  of 
the  coal  falling  in,  but  with  coke  this  is  not  always  neces- 
sary, instead  a  glowing  coal  may  be  laid  across  the  top 
of  the  crucible. 

Place  the  pots  in  one  unvarying  order,  so  that  any 
given  one  may  always  occupy  the  same  place  according 
to  its  number ;  thus  it  will  not  be  necessary  to  mark  them, 
though  this  may  also  be  done  by  means  of  reddle. 

If  more  assays  are  to  be  made  than  the  furnace  can 
contain  at  once,  a  second  set  should  be  prepared  before 
the  first  is  melted;  then,  as  soon  as  number  one  is  ready 
to  pour,  remove  it  carefully  from  the  fire,  preserving 
the  nest  which  it  occupied,  pour  it,  and  in  its  place  put 
the  first  of  the  new  set,  which  will  be  number  7,  if  the 
furnace  holds  6  pots,  and  so  on  with  the  others. 

If  time  is  an  object,  it  is  a  good  plan,  while  weighing 
and  fluxing  the  assays,  to  let  an  assistant  place  in  the  fire 
a  full  set  of  old  crucibles  as  dummies,  to  be  removed  one 
by  one  and  replaced  by  charged  pots  when  ready.  If  any 
of  the  assays  fail,  note  the  numbers,  and  make  them 
again  after  all  the  others  are  done ;  for  certainty,  celerity, 
and  convenience,  system  is  indispensable. 

The  assays  will  swell  when  heated,  hence  it  is  proper 
to  partly  or  wholly  uncover  the  pots  when  fusion  begins, 
and  to  watch  that  they  do  not  boil  over,  which  is  more 
likely  to  occur  if  they  are  closely  covered;  this  precau- 


AND   SILVER   ORES  'J'J 

tion  may  become  needless  when,  by  practise,  the  exact 
manner  of  fluxing  a  given  ore  so  as  to  avoid  the  danger 
is  known. 

The  tendency  to  boil  over  proceeds  from  several  causes, 
as  the  use  of  too  much  soda,  or  of  undried  borax,  the 
latter  especially,  with  slow  heating,  so  that  if  undried 
borax  is  used,  the  assays  may  be  put  at  once  into  a  strong 
heat;  but  this  can  not  be  done  when  much  niter  is  em- 
ployed, as  that  causes  foaming.  The  boiling  up  of  an 
assay  may  be  checked  by  throwing  in  a  teaspoonful  of 
salt.  It  sometimes  happens  that  an  assay  is  heated  too 
much  at  the  bottom  before  the  upper  part  is  hot,  produc- 
ing fusion  below,  while  a  crust  remains  above,  and  is 
pushed  out  of  the  pot  by  the  expanding  gases;  to  guard 
against  this,  place  the  pot  well  down  near  the  grate,  and 
pack  hot  coals  closely  about  the  upper  part. 

If  the  slightest  overflow  occurs,  or  if  the  fusing  mass 
comes  in  contact  with  the  cover,  the  assay  must  be  re- 
jected. If  the  slag  appears  thick,  try  the  addition  of  a 
little  borax;  if  that  does  not  help  it,  soda  will.  In  adding 
soda,  look  out  for  boiling  up.  A  certain  proportion  of 
soda  may  sometimes  make  a  thick  slag,  when  less  would 
work  well ;  borax,  glass,  or  litharge  is  the  remedy. 

When  the  fusion  is  finished,  in  from  five  to  twenty- 
five  minutes  after  subsidence,  the  pot  is  uncovered,  with 
care,  that  no  coal  may  fall  into  it,  seized  in  the  bent 
tongs,  removed  from  the  fire,  the  sides  rinsed  in  the  slag 
by  a  circular  swinging  movement,  tapped  on  the  edge  of 
the  furnace  to  settle  the  lead,  and  the  still  liquid  contents 
poured  into  the  mold.  The  pot  is  completely  inverted 
and  tapped  agiinst  the  mold.-  The  overflowing  of  the 
slag  in  the  mold  is  of  no  consequence.    The  pot  should  be 


78  ASSAYING    GOLD 

examined;  it  should  retain  only  a  glaze  of  slag,  and  if 
any  pasty  lumps  or  globules  of  lead  remain  in  it,  the 
assay  is  defective. 

If  nails  have  been  used  in  the  assay,  they  must  be  re- 
moved before  the  pouring.  With  tongs  seize  a  nail,  wash 
it  in  the  slag  and  tap  it  against  the  pot,  then  examine  it ; 
if  it  is  free  from  adhering  lead,  reject  it,  and  take 
another  the  same  way ;  if  lead  is  seen  to  stick  to  the  nail 
so  that  it  can  not  be  washed  off,  it  shows  that  the  assay  is 
not  finished,  either  for  want  of  heat  or  of  time,  and  it 
must  be  returned  to  the  furnace. 

When  cooled,  the  assay  is  turned  out  of  the  mold  and 
the  slag  examined ;  it  should  contain  no  globules  of  lead ; 
the  button  is  then  beaten  to  a  cube,  brushed,  marked,  if 
so  desired,  and  placed  in  readiness  for  cupellation. 

The  buttons  must  be  soft  and  malleable,  and  separate 
easily  from  the  slag  when  cooled.  The  color  of  the  slag 
varies  with  the  constituents  of  the  ore  and  the  propor- 
tions of  the  fluxes.  The  slag  is  not  always  glassy,  nor  is 
this  important,  but  it  must  be  liquid,  not  pasty,  when 
poured. 

Pots  which  have  not  been  cleanly  poured  may  be 
rendered  fit  for  re-use  by  melting  in  them  the  usual 
fluxes,  litharge,  etc.,  without  ore,  then  pouring. 


AND   SILVER   ORES  79 


SCORIFICATION. 


In  this  method  the  Utharge  required  for  fluxing  is 
made  from  lead  during  the  operation.  Scorification  is 
well  adapted  to  rich  ore,  but  not  to  that  which  is  poor, 
on  account  of  the  smallness  of  the  quantity  treated  or 
the  time  and  fuel  consumed;  however,  ores  containing 
nickel  or  tin  are  perhaps  better  scorified.  Buttons  con- 
taining tin  or  nickel  can  not  well  be  cupelled  until  scori- 
fied. Ores  are  scorified  with  an  addition  of  granulated 
lead  and,  generally,  borax. 

Granulated  lead,  like  litharge,  contains  silver,  and  must 
be  assayed.    Take — 

Granulated  lead,  2  or  4  A.  T. 
Powdered  glass,  a  little. 
Mix  in  a  scorifier.  Place  in  the  heated  muflie,  and  close 
the  door  until  the  lead  is  melted,  then  open  the  door 
and  maintain  a  moderate  red  heat,  keeping  the  lead  fused. 
When  the  lead  is  covered  by  the  slag,  remove  from  the 
muffle  and  pour  into  the  assay-mold.  Return  the  scori- 
fier to  the  muffle,  and  as  soon  as  the  assay  is  cool  enough 
to  separate  the  slag,  return  the  lead  to  the  scorifier,  and 
so  on  until  the  lead  is  of  a  suitable  size  for  the  cupel; 
weigh  the  resulting  bead,  and  note  its  weight  and  that  of 
the  lead  used  in  the  assay-book. 

Borax,  quartz,  or  glass  are  used  as  fluxes,  the  quan- 
tity varying  with  the  character  of  the  ore,  earthy  or  basic 
ores  requiring  much  more  than  the  acid  ores.  Too  much 
borax  causes  too  much  slag  at  first,  and  is  consequently 


8o  ASSAYING    GOLD 

often  introduced  later  in  the  operation.  This  excess  of 
slag  prevents  the  complete  decomposition  of  the  sul- 
phurets,  and  silver  remains  with  the  oxy-sulphurets  in 
the  slag. 

Ordinary  ores, require  from  8  to  12  parts  of  lead; 
galena,  or  silver  glance,  2  parts.  Those  containing  much 
iron,  zinc,  or  tin,  or  of  which  the  gangue  is  lime,  require 
more  lead,  sometimes  as  much  as  32  parts  to  one  of  ore. 
Nickel  and  cobalt  may  require  repeated  scorifications 
with  fresh  additions  of  lead  up  to  100  parts  before  a  soft 
button  is  got.  A  small  quantity  of  borax  is  beneficial, 
unless  the  gangue  is  quartz.  With  an  unfamiliar  ore., 
it  is  best  to  make  several  assays,  each  with  a  different 
quantity  of  lead;  if  that  one  in  which  the  greatest  quan- 
tity of  lead  was  used  gives  the  highest  result,  another 
should  be  made  with  yet  more  lead,  and  so  on  until  two 
assays  agree.  The  following  is  a  general  formula  for 
ordinary  ores: — 

Ore..: 0.2   A.    T. 

Lead 2.5 

and  a  little  dried  borax,  unless  the  gangue  is  quartzose.* 
Mix  one-half  of  the  lead  with  the  ore  in  the  scorifier ; 
cover  with  the  remaining  lead,  and  sprinkle  on  the  borax. 
Place  in  the  red-hot  muffle.  Close  the  door  until  the  lead 
is  thoroughly  melted.  Open  the  door,  and  moderate  the 
heat.  The  ore  will  be  seen  floating  upon  the  lead;  it 
soon  takes  the  form  of  a  ring,  leaving  the  lead  exposed; 
this  is  called  the  ''bull's  eye."  If  the  bull's  eye  does  not 
appear,  the  assay  will  fail,  unless,  perhaps,  with  an  oxi- 

*When  the  ore  contains  much  sulphur,  a  part  of  the  lead  may 
be  replaced  by  litharge. 


AND   SILVER   ORES  8l 

dized  ore;  this  is  sometimes  caused  by  the  ore  not  being 
finely  ground.  When  the  bull's  eye  appears,  maintain  a 
cupelling  heat  by  managing  the  damper  and  the  door  of 
the  ash-pit.  When  the  bull's  eye  is  covered  by  the  melted 
slag,  add  a  couple  of  lumps  of  borax  by  means  of  the 
cupel  tongs ;  close  the  door  and  give  a  higher  heat  for 
about  5  minutes.  When  the  slag  is  perfectly  fluid,  the 
assay  may  be  poured;  the  scorifier  should  be  returned 
to  the  muffle  (unless  it  is  much  corroded,  when  a  new  one 
must  be  taken),  because  the  button  may  be  too  large,  and 
must  then  be  rescorified.  The  button  is  cleaned  and  pre- 
pared for  cupellation  as  in  the  crucible-assay.  The  slag 
must  contain  no  prills  of  lead. 

When  very  sulphurous  ores  are  assayed,  it  is  best  to 
omit  the  borax  until  the  bull's  eye  is  closed.  The  scori- 
fiers  are  handled  by  means  of  the  scorifier-tongs',  and 
may  be  used  until  too  much  corroded,  though  the  same 
rules  apply  to  these  as  to  crucibles  in  regard  to  re-use. 
As  a  scorifier  may  occasionally  be  pierced  by  the  litharge 
formed,  it  is  well  to  have  an  extra  bottom  in  the  muffle ; 
such  a  bottom  may  be  bought,  or  made  by  cutting  another 
muffle.  The  muffle  bottom,  extra  or  not,  should  be  pro- 
tected by  a  thin  layer  of  bone-ashes,  which  will  absorb 
the  spillings  in  case  of  accident,  and  may  be  scraped  out. 
The  scorifiers  may  be,  in  a  measure,  protected  by  rubbing 
the  inside  with  reddle.  Scorification  furnishes  a  conve- 
nient means  of  purifying,  as  well  as  lessening  the  weight 
of,  a  button  got  in  the  crucible;  by  this  means,  also,  any 
substance,  except  the  noble  metals,  may  be  removed  from 
lead.  ^ 

Some  teachers  hold  that  scorification  is  the  only  re- 
liable method  for  the  assay  of  silver  ores,  while  others 


82  ASSAYING    GOLD 

contend  that  it  is  inferior  to  the  crucible-assay  when  the 
silver  is  present  as  chloride ;  others,  again,  maintain  the 
opposite  of  this.  In  the  case  of  chlorides,  I  usually  em- 
ploy a  little  soda  in  the  scorifier.  With  equal  quantities 
of  ore,  the  crucible  method  (first  system)  is  by  far  the 
quicker,  and  I  am  of  the  opinion  that,  if  enough  litharge 
be  used,  it  is  quite  as  accurate  as  scorification.  Scorifica- 
tion  appears  to  have  the  advantage  of  simplicity  in  its 
favor,  and  yet,  if  we  employ  an  abundance  of  litharge 
in  the  crucible,  as  we  must  of  lead  in  the  scorifier,  and 
omit  the  use  of  niter,  the  advantage  almost  or  quite  van- 
ishes, while  on  the  other  hand,  by  availing  ourselves  of 
the  oxidizing  power  of  niter,  in  the  case  of  sulphureted 
ores,  we  can  treat  a  considerably  larger  quantity  of  ore, 
in  proportion  to  the  weight  of  lead  to  be  cuj)elled  (or  re- 
scorified)  by  the  crucible  method  than  by  scorification. 
At  all  events  where  gold  is  to  be  determined  the  crucible 
is  preferable.  In  the  scorification  of  the  calcareous  ores, 
the  escaping  carbon  dioxide  gas  sometimes  causes  a 
serious  loss  of  substance  by  projection. 

The  following  reactions  express  briefly  the  chemistry 
involved  in  the  diflferent  processes  of  scorification  of 
difi^erent  ores : — 

Galena  ores : 

Pb+0=PbO  ] 

PbS+30=PbO+S02  \      Low  heat. 

PbS+40=PbS0,  J 

PbS-f  PbS04=2Pb-f2S0,       1  Tj.   r.  1,     ,   . 

Pbs+2Pbo=3Pb+sa  "    }  ^^^^  ""'''• 


AND   SILVER   ORES 


83 


For  sulphide  ores  (iron  pyrites)  : — 
2FeS2+iiO=Fe203+4S02  (roasting) 
'      3FeS2+X  PbO=3FeO,  X  PbO    (slag)  +6S02+Pb 

Copper  ores  : —    , 

CuS^+Pb^PbS,   CuS   (melting) 

PbS  CuS+PbO=PbCu+S02  (roasting) 

Pb+2Cu+PbO=Cu,0,  PbO+Pb 

Similarly  for  arsenic  and  antimony  ores. 

Tellurides : — 

Au2Te+2PbO=Te02+2AuPb 
If  the  PbO  be  placed  on  top  of  the  charge,  it  reacts  with 
the  AugTe  fumes  and  retains  the  gold. 

A  TABLE  OF  SCORIFICATION  CHARGES 


Ore 

A.  T. 

Lead  in 
grams 

Borax  Glass,  Etc. 

Average  Ores 

Va 

45 

54  Gram 

Cu  Sulphides 

Vxo 

75-90 

V2       " 

Cu  Matte 

Vio 

90-100 

%  SiO^-f  ^  Borax 

Fahl  Ores 

Vxo 

60-75 

^  Borax 

Pyrite 

Vxo 

45 

Ya     " 

Fe^Oa 

'% 

50 

y2  SiO^-f-H  Borax 

Galena 

/2 

45 

%  Borax-f  Fe  Nail 

Zinc  Blende 

V5 

100 

^  Borax+a  little  C 

Arsenic 

Va 

100 

1)^  Borax  Glass 

Tellurides 

Vxo 

50 

10  gms.  PbO  on  top-f-^  Borax 

Rich  Au  &  Ag  Ores 

Vxo 

45 

y2  Borax  Glass 

84  ASSAYING    GOLD 


CUPELLATION. 


By  this  process  lead  is  separated  from  precious  metal. 
The  cupel  should  be  somewhat  heavier  than  the  button  ;* 
it  is  placed  in  the  brightly  red-hot  muffle  by  means  of  the 
cupel-tongs,  and  the  door  is  closed.  When  the  cupel  is 
thoroughly  heated,  the  button  is  placed  upon  it.f  Again 
the  door  of  the  muffle  is  closed,  until  the  lead  is  well 
melted,  when  it  is  opened  to  admit  air.  The  lead  now  ap- 
pears luminous.  The  heat  and  the  draft  through  the 
muffle  must  be  regulated  by  means  of  the  damper  and 
the  ash-pit  door,  so  that  the  lead  smoke  rises  to  about  the 
middle  of  the  muffle.  The  button  gradually  becomes 
smaller,  and  colored  spots  are  seen  moving  upon  its  sur- 
face; these  spots  are  litharge  which  sinks  into  the  cupel. 
The  heat  should  now  be  reduced;  not  so  much  that  the 
button  shall  seem  to  swim  in  litharge,  but  so  that  a  crop 
of  small  crystals  of  litharge  shall  be  formed  on  the  cupel. 
After  a  time  the  button  appears  covered  by  rainbow- 
colored  rings,  which  seem  to  move  rapidly ;  it  becomes  a 
''bead"  which  seems  to  rotate  upon  its  axis;  at  this  in- 
stant the  cupel  is  pushed  further  back,  where  the  heat  is 


*A  cupel  will  absorb  about  twice  its  weight  of  litharge,  but  if  it 
is  saturated,  the  bead  may  not  be  quite  clean,  and  the  litharge 
may  injure  the  muffle. 

fThe  cupel-tongs  are  used  here ;  they  may  be  steadied,  if  neces- 
sary, by  contact  with  the  edge  of  the  muffle.  I  always  place  a 
few  small  pieces  of  charcoal  in  the  front  end  of  the  muffle,  in 
order  to  equalize  the  heat. 


AND   SILVER   ORES  85 

greater,  or  a  glowing  coal  is  placed  close  to  it.  Suddenly 
the  bead  becomes  still,  and  in  a  few  .seconds  more  the 
ciipellation  is  finished.  The  finishing  is  called  the 
brightening  or  blick. 

If  the  bead  consists  largely  of  gold,  which  is  known 
by  its  greenish  color  while  fused,  or  if  it  is  not  larger 
than  the  head  of  a  large  pin,  the  cupel  may  now  be  re- 
moved from  the  muffle ;  not  so  with  the  larger  bead,  con- 
sisting principally  of  silver.  Melted  silver  absorbs  oxy- 
gen from  the  air,  and  expels  it  while  cooling;  if  a  large 
bead  is  cooled  suddenly  on  the  surface,  while  the  cupel 
is  still  highly  heated,  the  gas,  when  expelled  from  the 
interior,  breaks  forcibly  through  the  solidified  crust,  car- 
rying with  it  some  molten  silver,  a  part  of  which  is  lia- 
ble to  be  lost.  Such  an  accident  may  be  prevented  by 
causing  the  bead  to  solidify  very  slowly,  or  from  below 
upward ;  this  is  done  in  either  of  two  ways  ■  First,  by 
inverting  over  the  bead  on  the  cupel  another  cupel  heated 
to  redness ;  second,  and  more  surely,  by  drawing  the 
cupel  toward  the  front  of  the  muffle,  watching  it,  tongs  in 
hand,  and  at  the  instant  when  the  bead  ceases  from 
trembling  on  the  cupel  being  lightly  tapped  on  one  side 
by  the  tongs,  pushing  the  cupel  back  to  the  hotter  part 
of  the  muffle.  The  surface  of  the  bead  is  thus  prevented 
from  solidifying,  or  is  again  melted,  while  the  partially 
cooled  cupel,  absorbing  the  heat,  causes  the  solidifica- 
tion  to  proceed  from  below,  the  gas  escaping  quietly.  The 
disruption  of  the  bead  as  described  is  called  sprouting  or 
vegetation.  At  the  instant  of  solidification,  the  bead 
suddenly  emits  a  flash  of  light. 

The  signs  of  a  successful  cupellation  are  as  follows: 
Some  crystals  of  litharge  are  seen  on  the  cupel ;  the  bead 


86  ASSAYING    GOLD 

is  well  rounded,  bright  on  the  upper  surface,  unless  when 
slightly  frosted  by  incipient  sprouting,  or  by  being  kept 
too  long  in  a  fused  state  after  the  blick,  crystalline  below, 
adhering  slightly  to  the  cupel.  A  bead  which  does  not 
adhere  a  little  to  the  cupel  contains  lead;  too  strong  ad- 
herence indicates  that  the  button  contained  other  base 
metal  and  not  enough  lead  for  its  complete  removal;  the 
bead  then  probably  contains  copper.  In  either  of  these 
cases  the  bead  must  be  enveloped  in  a  small  piece  of  sheet 
lead  and  recupelled.  As  several  cupellations  may  be  con- 
ducted at  once,  the  cupels  may  be  marked  by  vertical 
scratches    on   the    sides,    in    order   to    avoid   mistakes.* 

*The  writer  never  marks  the  cupels,  but  places  them  on  the  tray 
and  in  the  muffle  in  one  unvarying  order,  according  to  their  num- 
bers. When  many  assays  are  made  at  once  it  is  best  to  work 
so  that  those  in  front  shall  be  finished  first,  in  order  that  the 
finishing  may  be  managed,  and  the  cupels  removed  easily  from  the 
muffle  at  the  proper  time,  for  if  left  in  long  after  the  brightening 
a  large  loss  of  silver  may  result.  If  the  buttons  are  all  of  one  size 
there  can  be  no  objection  to  charging  them  all  on  to  the  cupels  as 
soon  as  the  latter  are  hot  enough ;  otherwise,  small  buttons  which 
may  from  their  numbers  fall  to  the  rearward  cupels,  should  be 
reserved  until  the  larger  ones  nearer  to  the  front  have  made 
some  progress. 

If  for  any  reason  the  cupels  are  placed  out  of  the  regular  order 
of  position  in  the  muffle,  a  chart  on  a  piece  of  paper  should  be 
made  showing  the  situation  of  each  number. 

In  important  assays  of  rich  ore  or  bullion  only  so  many  cupels 
should  be  worked  at  one  time  as  can  be  moved  back  or  forward, 
etc.,  at  will. 

Finally,  in  poor  and  comparatively  unimportant  assays,  extreme 
particularity  is  neither  requisite  nor  feasible.  The  main  point  is  to 
guard  against  excessive  heat.  Practise  makes  perfect,  and  rules 
are  for  tyros ;  so  take  some  test  silver,  and  lead,  and  work  away 
on  cupellation,  noting  the  losses,  until  you  find  out  what  you 
can  do. 


AND   SILVER   ORES  87 

Each  set  of  assays  is  numbered  i,  2,  3,  etc.,  irrespective 
of  the  numbers  of  the  samples  or  deposits  from  which 
they  are  made,  which,  however,  must  not  be  lost  sight  of. 
On  removal  from  the  muffle,  the  cupels  are  placed  on  the 
cupel-tray  and  carried  to  the  small  anvil.  The  larger 
beads  are  seized  by  the  flat-nosed  plyers,  laid  on  the 
anvil,  again  grasped  well  within  the  jaws  of  the  plyers, 
and  forcibly  compressed;  this  loosens  any  adhering  lith- 
arge or  bone-ash,  which  is  then  removed  by  brushing. 
Very  small  beads  are  removed  by  the  pincettes,  laid  upon 
the  anvil,  turned  on  the  side,  and  flattened  by  a  blow  from 
the  small  hammer,  which  has  the  same  effect  as  the  com- 
pression in  the  plyers.  Extremely  small  beads  may  be 
rolled  on  the  anvil  under  the  face  of  the  hammer;  these 
should  be  examined  by  the  aid  of  the  lens,  to  see  that 
they  are  clean,  and,  if  necessary,  rubbed  between  paper. 
Each  cleaned  bead  is  placed  on  a  watch-glass  bearing 
the  set  number  of  the  assay  on  a  disk  of  paper  pasted  to 
the  convex  side,  so  that  it  can  be  read  through  the  glass. 
The  glasses  are  ranged  on  a  tray  similar  to  the  cupel- 
tray.f 

Some  assayers  replace  the  cleaned  beads  on  the  cupels, 
and  thus  carry  them  to  the  assay-balance;  it  is  a  ques- 
tionable practise,  and  leads  to  the  occasional  loss  of  a 
bead.  If  the  cupel  is  placed  within  the  scale-case,  the 
result  is  dirt;  if  not,  the  bead  is  usually  carried  some 
distance  in  the  forceps,  and  is  liable  to  be  dropped  to  the 
floor,  where  a  very  small  bead  can  not  be  found.  The 
watch-glass  containing  the  bead  is  placed  close  to  the  pan 


fThe  dry-cups  may  be  used  instead  of  watch-glasses  and  need 
not  be  numbered  if  placed  in  regular  order. 


88  ASSAYING    GOLD 

of  the  balance,  and  loss  of  the  bead  is  scarcely  possible, 
or  the  beads  are  removed  from  the  glasses  and  placed  in 
a  row  within  the  scale-case,  then  weighed  in  succession. 
The  weight  of  each  bead  is  noted  in  a  book  kept  for 
the  purpose,  opposite  to  the  set  and  sample  numbers; 
under  the  weight  is  noted  the  correction  due  for  silver 
in  the  litharge  or  lead  used  in  the  assay,  which,  being 
subtracted,  leaves  the  net  weight  of  precious  n:>etal  ob- 
tained from  the  ore.  In  assaying  an  ore  which  is  likely 
to  contain  no  silver  worth  notice,  it  is  proper  to  add  a 
small  piece  of  gold-free  silver  to  the  button  for  cupella- 
tion,  in  order  to  insure  a  bead  of  convenient  size,  and  to 
prevent  a  possible  loss  of  gold  by  its  sinking  into  the 
cupel;  if  the  added  silver  is  first  weighed,  any  silver 
which  the  ore  may  yield  can  be  determined. 

Note. — There  is  always  some  silver  lost  in  cupelling,  but  it  is 
not  usual  to  make  a  correction  except  in  the  assay  of  bullion. 
See  Part  II.  The  loss  may  be  approximately  ascertained  by  re- 
cupelling  the  bead,  before  parting,  with  a  weight  of  lead  equal  to 
that  of  the  original  button.  The  loss  of  weight  in  the  second 
cupellation  will  give  an  idea  of  that  in  the  first;  but  as  the  heat 
used  and  time  occupied  may  be  different,  the  loss  may  also  differ 
materially  in  a  large  bead.  A  better  way  is  to  smelt  out  another 
button,  add  enough  lead  to  the  first  bead  to  make  a  button  of 
equal  weight  with  the  new  one,  and  cupel  the  two  side  by  side. 
They  will  then  lose  equally,  as  nearly  as  may  be,  and  the  dif- 
ference of  their  weights  added  to  that  first  got  will  be  the  truth, 
and  will  be  equal  to  the  weight  of  the  second  bead  plus  the  loss 
sustained  by  the  first  in  recupellation.    Thus : — 


AND  SILVER  ORES  89 

Weight  of  first  bead 347 

Weight  of  second  bead 346 

First   bead    recupelled 342  342 

Differences   4  5 

Old  bead 347  new  346 

True  weight. 351  351 


90  ASSAYING    GOLD 


WEIGHING  THE  BEAD. 


When  about  to  weigh  a  bead,  the  operator  must  first 
see  that  the  assay-balance  is  in  equilibrium ;  if  this  is  not 
the  case,  dusting  by  means  of  the  camel's-hair  brush,  or 
wiping  the  capsules  with  a  fine  handkerchief,  will  fre- 
quently make  it  so;  the  extra  supports  may  also  require 
to  be  cleaned  occasionally,  as  they  sometimes  adhere 
slightly  to  the  beam  or  pans,  drawing  one  side  down 
when  lowered.  When  the  weight  of  a  bead  or  other  ob- 
ject is  required,  it  is  best  placed  on  the  left-hand  pan,  be- 
cause the  right  hand  is  then  conveniently  employed  in 
adjusting  the  weights  on  the  right-hand  pan,  while  the 
left  hand  operates  the  turnkey  by  which  the  supports  are 
raised  or  lowered.  (In  some  balances  the  extra  supports 
are  fixed,  and  the  planes  on  which  the  beam  rests  when 
in  action  are  movable ;  the  principle  is  the  same. ) 

When  equilibrium  is  nearly  attained,  the  case  must  be 
closed  at  each  trial,  because  the  slightest  current  of  air 
will  disturb  the  action  of  the  balance;  it  is  here  that  the 
great  convenience  of  the  riders  and  carriers  is  realized. 
The  weights  having  been  adjusted  to  the  nearest  lo,  or 
lower,  if  so  preferred,  the  case  is  closed,  and  the  operation 
finished  by  placing  the  rider  on  the  beam  at  that  point 
at  which  it  produces  equilibrium.  It  is  better  that  the 
beam  should  swing  a  little,  as  that  proves  it  to  be  free; 
it  may  be  caused  to  do  so,  when  necessary,  by  a  gentle 
touch  of  the  carrier.     An  object  should  not  be  weighed 


AND   SILVER   ORES  9 1 

while  it  is  hot,  as  currents  of  air  would  be  produced 
which  would  make  it  appear  too  light. 

It  may  be  known  when  the  balance  is  in  equilibrium 
without  waiting  for  the  beam  to  come  to  rest,  which, 
with  a  very  sensitive  instrument,  would  be  tedious.  The 
oscillations  diminish  continually,  so  that  no  two  will  be 
exactly  alike,  but  if  the  mean  of  two  swings  to  one  side 
is  sensibly  equal  to  the  intermediate  swing  to  the  other 
side  of  zero,  the  balance  is  in  equilibrium.  When  riders 
are  not  used,  the  fractions  of  the  unit  may  be  estimated 
by  the  oscillations  of  the  pointer ;  they  will  be  of  greater 
amplitude  on  one  side  of  zero  than  on  the  other,  and  the 
middle  point  between  any  two  consecutive  extremes  is 
very  nearly  that  at  which  the  pointer  would  come  to  rest 
if  allowed  to  do  so.  More  accurately,  the  middle  point 
between  the  mean  of  two  deviations  on  one  side  and  the 
intervening  deviation  on  the  other  side  of  zero  is  that 
at  which  the  pointer  would  rest.  If,  for  example,  the 
pointer  swings  5  divisions  to  the  right,  then  3  to  the 
left,  and  again  4  to  the  right,  the  mean  between  4  and  5 
is  4>^,  and  the  middle  point  between  4^  to  the  right  and 
3  to  the  left  is  ^ths  to  the  right;  or  the  beam  may  be 
allowed  to  come  to  rest  and  the  deviation  noted. 

The  value  of  this  amount  of  deviation  must  be  found 
by  calculation  from  the  observed  deviation  caused  by  a 
known  preponderance  of  weight.  Thus  if  it  has  been 
found  by  trial  that  the  one  weight  causes  the  pointer  to 
stop  at  5  divisions  from  zero,  the  value  of  i  division  is 
0.2  and  of  ^ths  of  a  division  is  0.15.  In  any  case,  the 
swing  is  useful  as  an  indication  of  the  weight  required, 
and  the  assayer  should  study  the  action  of  his  instrument 


92  ASSAYING    GOLD 

under  different  loads.     The  bead,  having  been  weighed, 
must  be  parted,  unless  the  ore  is  known  to  carry  no  gold. 

There  are  two  methods  of  weighing  by  the  oscillations,  and 
this  fact,  together  with  the  indefinite  manner  in  which  the  terms 
designating  the  position  and  movements  of  the  pointer  are  used 
by  different  writers  and  operators,  has  caused  some  confusion 
and  misconception  among  students. 

The  one  method  is  to  take  the  excess  of  movement  on  either 
side  of  zero  as  the  indication  of  the  excess  of  weight ;  th'e  other  is 
to  take  the  distance  from  zero  at  which  the  pointer  will  finally 
rest  as  such  indication.  The  first  method  will  give  twice  as  many 
divisions  of  the  arc  as  the  second.  Hence  the  value  of  a  division 
is  but  half  as  much  in  the  first  as  in  the  second. 

In  order  to  clear  the  subject  of  all.  ambiguity,  I  propose  the  fol- 
lowing terms :  Range,  swing,  and  rest.  Range  is  the  total  move- 
ment of  the  index,  counted  in  divisions  of  the  arc  (incorrectly 
called  vernier  by  some).  Range  may  extend  to  each  side  of  zero, 
and  is  then  written  +  ^^^  — >  plus  being  usually  the  right-hand 
side,  minus  the  left,  as  +  5  —  3.  Or  range  may  extend  from  any 
division  on  either  side  of  zero ;  as  +  5  —  0,  or  —  6  -f-  0-  Again 
the  pointer  may  move  to  either  side  and  not  return  to  zero,  as 
-f-7-f-3,  that  is,  the  pointer  goes  to  -\-7  and  comes  back  to  +3,  or 
— 7 — 3  on  the  other  side.  Range  has  no  value  as  an  indication  of 
the  amount  of  preponderance,  but  it  affords  a  means  of  calculating 
swing  and  rest. 

Swing  is  the  excess  of  movement  to  one  side  or  the  other  of 
zero,  caused  by  a  preponderance  of  weight  on  one  of  the  pans; 
hence  a  balance  in  equilibrium  will  have  range,  if  the  beam  be  set 
in  motion,  but  can  have  no  swing  in  the  sense  which  I  propose  for 
that  term.  Swing  is  equal  to  range  when  one  extreme  of  range  is 
zero;  it  is  the  difference  of  the  distances  from  zero  when  the 
pointer  goes  to  each  side  of  zero,  that  is,  when  the  signs  of  the 
distances  are  opposite;  it  is  the  sum  of  the  distances  when  the 
signs  are  similar,  because  if  the  pointer  marks  +5  and  comes 
back  to  -1-3,  it  would  mark  -f8 — 0,  if  the  beam  had  impetus 
enough,  or  -\-9 — 1,  etc.,  in  each  instance  giving  swing  =  8. 

Range  is  a  matter  of  observation  only.    Swing  may  be  a  matter 


AND   SILVER   ORES  93 

of  obsenvation,  for  if  impettrs  enough  be  given  to  the  beam  to 
make  the  pointer  move  to  right  and  left  of  zero,  a  time  must  come 
when  the  gradual  decrease  of  movement  will,  practically  speaking, 
cause  it  to  touch  zero  at  one  extreme,  and  when  that  is  the  case, 
as  said  above,  swing  is  equal  to  (identical  with)  range.  But 
swing  may  be  calculated  from  observed  range  at  any  time  when 
the  beam  is  oscillating,  as  shown  above. 

Rest  is  the  distance  from  zero  at  which  the  pointer  will  stop  if 
allowed  to  do  so.  In  this  sense  a  balance  in  equili^ium  can  have 
no  rest,  though,  as  before  remarked,  it  may  have  range.  Rest  is 
always  the  half  of  swing;  it  may  be  observed  by  waiting  for  it,  or 
it  may  be  known  in  advance  by  finding  swing  and  dividing  that 
by  2. 

Swing,  and  therefore  rest,  may  be  deduced  from  a  single  obser- 
vation of  range,  but  more  accurately  from  the  mean  of  several, 
thus:  Note  on  a  piece  of  paper  three  (or  more)  extremes  in  one 
direction,  and  the  intervening  two  (or  more)  extremes  in  the 
other  direction,  prefixing  to  each  its  appropriate  sign ;  find  swing 
by  adding  the  means  together  if  the  signs  are  similar,  subtracting 
the  lesser  from  the  greater  if  the  signs  are  opposite.  In  the  first 
case  the  sign  of  swing  will  be  the  same  as  that  of  range;  in  the 
second,  it  will  be  that  of  the  greater  quality.  Rest  takes  the 
same  sign  as  swing. 

Range  may  be  increased,  whenever  desired  for  convenience,  by 
wafting  with  the  hand  a  slight  current  of  air  against  either  of  the 
pans,  after  which  the  case  must  be  closed,  and  a  few  moments 
allowed  to  elapse  before  beginning  the  observation.  Examples: 
Range  (an  average)  is  +8 — 3  as  the  signs  are  opposite,  swing  is 
the  difference,  =  -|-5 ;  rest  is  +2j^.  Again ;  range  is  — 8-|-3 ;  the 
signs  are  opposite  and  swing  is  — 5;  rest,  — 2y2.  Let  range  be 
-f-  7  -\-2\  the  signs  are  similar  and  swing  is  the  sum  =1  -j-  9;  rest  is 
+45^.  If  range  is  -|-6 — 0,  swing  is  -f 6,  and  rest  is  +3;  or  if 
range  be  — 6-|-0,  swing  is  — 6,  rest  — 3. 

The  value  of  swing  or  rest  for  the  balance  in  use  may  be 
found  by  trial  with  a  known  preponderance  of  weight  on  either 
pan;  it  varies  slightly  with  the  load  in  the  pans.  Whether  the 
preponderance  of  weight  indicated  by  swing  or  rest  must  be 
added  to  or  subtracted  from  the  weights  in  the  weight-pan  must 


94  ASSAYING    GOLD 

be  left  to  the  common  sense  of  the  workman,  since  it  depends 
upon  which  pan  contains  the  object  to  be  weighed.  If,  however, 
the  signs  are  used  as  here  suggested,  and  the  object  be  on  the 
left-hand  pan,  and  the  index  of  the  balance  points  downward,  the 
value  of  a  -f-  swing  or  rest  must  be'  added,  that  of  —  subtracted. 

Weighing  by  the  oscillations  is  useful  when  riders  are  not  used, 
and  when  a  balance  is  not  very  sensitive. 

Weighing  by  the  riders  is  much  more  rapid  than  by  the  oscil- 
lations, but  it  requires  a  more  sensitive  balance.  The  sensitiveness 
of  a  rider  balance  should  be  at  least  equal  to  1-10  of  a  milligram, 
or  preferably  4-100  of  a  milligram  for  orle  division  of  the  arc. 


AND   SILVER   ORES 


PARTING. 


95 


Silver  is  soluble  in  nitric  acid;  gold  is  not,  hence,  if 
we  boil  an  alloy  of  gold  and  silver  in  nitric  acid,  the 
silver  is  dissolved,  the  gold  remains;  but  in  order  that 
the  separation  may  be  complete,  it  is  necessary  that  the 
alloy  shall  contain  at  least  twice  as  much  silver  as  gold, 
and  that  it  be  in  the  form  of  a  thin  sheet,  unless  it  con- 
tains a  much  larger  proportion  of  silver.  With  from  2 
to  3  parts  of  silver  to  i  of  gold,  the  separation  is  perfect 
(practically  speaking),  and  the  gold  remains  in  a  single 
piece ;  with  a  larger  proportion  of  silver  the  gold  re- 
mains as  a  dark  powder.  The  beads  obtained  from  an  ore 
assay  frequently  contain  a  very  large  proportion  of  silver, 
and  are  then  at  once  boiled  with  nitric  acid  in  a  test-tube 
until  no  more  red  fumes  are  seen  and  the  particles  of 
gold  have  been  collected  together,  when  the  acid  is  care- 
fully poured  off;  the  tube  is  then  nearly  filled  with  pure 
water,  the  gold  allowed  to  settle,  and  the  water  poured  off 
(usually  twice) .  The  tube  is  now  filled  with  water  and  held 
between  the  fingers  of  the  right  hand;  a  dry-cup  is  in- 
verted over  it  and  retained  by  the  thumb.  By  turning  the 
wrist,  the  tube  is  inverted,  the  gold  settles  through  the 
water  to  the  bottom  of  the  dry-cup,  which  is  now  right 
side  up,  the  mouth  of  inverted  tube  within  it.  When 
the  gold  has  all  settled,  the  dry-cup  is  held  by  the  left 
hand,  the  tube  by  the  right,  and  the  latter  is  carefully 
raised,  allowing  air  to  enter  and  water  to  escape,  nearly 
filling  but  not  overflowing  the  cup.     A  pause,  to  allow 


g6  ASSAYING    GOLD 

the  disturbed  gold  to  again  subside,  and  by  a  dexterous 
movement,  like  breaking  it  off,  the  tube  is  removed  from 
the  cup,  at  the  same  time  being  turned  mouth  upper- 
most. The  spilling  of  some  water  from  the  tube  is  of  no 
consequence,  provided  it  does  not  overflow  the  cup,  by 
which  gold  would  be  lost,  but  a  skillful  operator  will 
scarcely  spill  a  drop.  The  cup  now  contains  water  and 
the  gold;  it  is  gently  tapped  and  manipulated  until  any 
separated  particles  of  gold  are  gathered  together;  the 
water  is  then  poured  off.  In  each  pouring  off  of  acid  or 
water  from  the  gold,  a  glass  rod,  test-tube,  or  dry-cup,, 
is  held  to  the  lip  of  the  vessel  containing  th^  gold ;  down 
this  the  liquid  flows  steadily,  without  danger  of  loss  of 
the  metal.  The  gold  remains  in  the  cup,  is  drained  as 
much  as  possible,  and  any  remaining  water  is  absorbed  by 
the  porous  dry-cup.  The  dry-cups,  marked  with  reddle 
if  so  preferred,  and  containing  the  gold,  are  placed  on 
a  tray  and  taken  to  the  muffle,  in  which  they  are  placed 
by  means  of  the  cupel-tongs  (or  a  special  pair  with  cir- 
cular jaws),  and  heated  to  redness,  again  placed  on  the 
tray,  and,  when  cool,  carried  to  the  assay-balance,  and 
the  contents  of  each  weighed  in  its  regular  order.  The 
gold  usually  adheres  slightly  to  the  dry-cup;  it  must  be 
loosened  by  the  point  of  the  fine  steel  pincettes,  or  by 
means  of  a  needle,  etc.,  and  transferred  to  the  weighing 
capsule  by  inverting  the  dry-cup  over  that  and  tapping 
the  cup.*     The  weight  of  the  gold  is  noted  under  the 


*It  is  a  good  plan  to  examine  with  a  lens,  and  remove  any 
foreign  particles  by  touching  them  with  the  moistened  point  of  a 
needle,  to  which  they  will  adhere.  A  sail-needle  with  the  eye  end 
inserted  into  a  cork,  which  serves  as  a  handle,  is  a  convenient 
instrument. 


AND   SILVER   ORES  97 

net  weight  of  the  appropriate  bead,  from  which  it  is  sub- 
tracted, leaving  the  net  weight  of  the  silver. 

To  those  beads  which  do  not  contain  enough  silver  for 
the  parting,  more  must  be  added.  The  cases  in  which 
this  addition  must  be  made  are  as  follows : — 
•  I.  If  the  bead  is  yellow,  it  requires  the  addition  of  2 
or  23^  times  its  weight  of  silver,  according  as  it  is  pale 
yellow  or  darker. 

2.  If,  on  attempting  the  parting,  the  bead  is  found  to 
be  blackened,  but  not  much  attacked,  it  requires  the  ad- 
dition of  2  parts  of  silver. 

3.  If,  after  parting,  the. weight  of  the  gold  is  found 
to  be  more  than  one-third  of  that  of  the  bead,  it  must 
have  as  much  as  25^  times  its  weight  of  silver  added,  and 
be  again  parted. 

This  addition  of  silver  is  called  inquartation.  For 
this  purpose  some  test  silver,  cut  in  small  pieces,  is  kept 
on  a  watch-glass  in  the  scale-case.  For  small  beads 
the  silver  is  not  always  weighed,  but  a  piece  is  selected 
which  is  known  to  be  sufficient.  It  is  better  weighed; 
the  gold  can  then  be  kept  in  one,  or,  at  most,  two  or  three 
pieces,  and  there  is  less  danger  of  loss  in  the  washing. 

The  silver  and  the  bead  are  placed  in  a  cornet,  or 
capsule,  made  by  folding  a  small  piece  of  thin  sheet 
lead  in  the  shape  of  a  hollow  cone,  which  is  then  closed 
and  folded  as  a  little  pocket ;  this  is  cupelled  and  the 
bead  cleaned  as  before,  flattened  on  the  anvil,  annealed 
by  heating  to  redness,  parted,  washed,  "cupped,"  heated 
to  redness,  and  weighed.  If  the  gold  does  not  fall  to 
powder  in  the  parting,  it  should  be  boiled  a  second  time 
for  several  minutes  in  fresh  acid  before  cupping.  The 
flattened  bead  can  be  annealed  on  a  piece  of  charcoal 


98  ASSAYING    GOLD 

before  the  blowpipe  flame,  or  on  a  clean  cupel  in  the 
muffle. 

For  parting  a  bead  not  larger  than  a  pin's  head,  a 
%-inch  tube  is  suitable;  for  a  larger  bead,  a  larger  tube 
should  be  taken.  The  depth  of  the  acid  in  the  tube 
should  not  much  exceed  the  diameter  of  the  tube,  for 
safety  against  bumping.  Heat  but  slightly  until  the 
action  diminishes,  and  then  boil  gently.  It  is  best  to 
keep  the  tube  in  motion,  shaking,  as  that  tends  to  pre- 
vent bumping.  A  small  quantity  of  gold  may  easily 
escape  observation  in  a  test-tube,  unless  the  boiling  be 
continued  until  the  acid  is  colorless  and  the  particles 
of  metal  gather  together;  by  then  pouring  off  the 
greater  part  of  the  acid,  adding  a  fresh  quantity,  and 
again  boiling,  the  gold  is  obtained  in  a  more  compact 
state,  better  for  the  washing  and  cupping.  The  mouth 
of  the  tube  should  be  directed  away  from  the  person 
during  the  boiling.  In  looking  for  the  gold,  hold  the 
tube  over  a  white  object. 

Some  assayers  prefer  to  use  a  small  glazed  porcelain 
capsule  for  the  parting ;  cupping  is  then  unnecessary,  the 
gold  being  washed  and  dried  in  the  capsule.  If  the 
parted  gold  is  scattered,  a  lively  stirring  round  with 
glass  rod  will  cause  it  to  collect.  As  the  glazed  capsule 
does  not  absorb  the  moisture,  it  must  not  be  heated 
strongly  until  the  gold  is  dried,  otherwise  the  boiling 
of  the  water  may  scatter  the  particles  of  metal,  or  even 
throw  them  out.  Wipe  the  interior  of  the  basin  care- 
fully with  a  small  piece  of  filter-paper,  but  avoid  touch- 
ing the  gold,  then  dry  below  boiling  heat;  finally  heat 
to  lozv  redness.  Too  high  heat  will  cause  the  glaze  to 
soften.     The  method  is  convenient  when  many  assays 


AND   SILVER   ORES  99 

are  to  be  made,  because  a  number  can  be  parted  at  the 
same  time  on  the  sand-bath.  However,  small  flat-bot- 
tomed matrasses  might  be  used  equally  well  on  a  sand- 
bath,  or  an  arrangement  for  supporting  a  number  of  test- 
•  tubes  in  an  oblique  position,  which  is  better  than  the 
vertical. 

The  use  of  the  Kennedy  flask  is  advisable  where  there 
is  possibility  of  the  gold  breaking  up.  They  are  heated 
on  a  sand-bath  or  similar  contrivance. 

In  the  parting  it  has  been  found  best  to  use  acid  of 
two  strengths,  first  dissolving  the  silver  in  nitric  acid 
of  1. 1 6  specific  gravity,  and  after  washing,  finishing 
with  acid  of  1.26  specific  gravity.  This  insures  that 
all  the  silver  is  dissolved,  with  less  danger  of  breaking  up 
the  gold. 

Another  excellent  way  is  to  place  the  flattened  bead  in 
the  test-tube,  or  other  receptacle,  fill  to  the  required 
depth  with  water  alone,  add  a  drop  or  two  of  strong 
nitric  acid,  and  heat  gently.  If  action  commences  the 
bead  will  part;  if  there  is  no  action  on  continued  heat- 
ing and  the  addition  of  another  drop  or  two  of  the 
strong  acid,  pour  off  the  solution,  wash,  dry,  and  alloy 
with  twice  its  weight  of  silver.  Flatten  and  proceed  as 
before.  When  action  is  complete,  pour  off  the  spent 
acid,  and  add  a  fresh  solution  of  strong  acid,  and  heat 
to  boiling.  Boil  for  one  minute,  then  pour  off  the  acid, 
wash  two  or  three  times,  dry  and  anneal.  If  the  action 
of  the  first  acid  is  too  violent,  dilute  with  water.  By  this 
method  a  bead  may  always  be  parted  in  one  piece,  no 
matter  what  the  proportion  of  silver  to  gold. 

Chemically-pure  concentrated  nitric  acid  has  a  spe- 
cific   gravity    of    1.42.      To    save   time,    many    assayers 


100  ASSAYING    GOLD 

use  but  one  acid,  of  1.20  specific  gravity,  one  part  of 
water  to  one  of  acid.  In  general,  to  determine  the  ratio 
of  water  to  acid  in  making  up  acid  of  any  desired 
specific  gravity  for  parting,  let  1.42  be  the  acid  strength 
and  i.xy  be  the  desired  strength.  Then,  ^2 — j_yQ|_ 
ume  of  water  necessary  to  add  to  one  volume  of  acid. 


AND   SILVER   ORES  lOI 


CALCULATING    THE    ASSAY. 


The  weights  of  gold  and  silver  are  reported  as  already 
said,  in  units  and  decimal  fractions  of  the  system  of 
weights  in  use.  The  best  system  is  the  gram  or  metric 
system.  Gold  values  are  reported  as  so  many  dollars 
per  ton,  or  as  so  many  Troy  ounces  (i  Troy  ounce 
gold  worth  $20.6718)  per  ton.  Silver  values  are  best 
reported  as  so  many  Troy  ounces  per  ton.  There  is 
likely  to  be  some  confusion  if  this  method  is  not  adopted 
for  silver,  because  the  assay  value  of  silver  is  $1.2929  per 
Troy  ounce,  being  the  value  it  had  some  years  ago  as 
compared  with  gold ;  at  present  silver  is  worth  as  a  com- 
modity, less  than  half  that,  and  the  value  thus  found 
must  be  discounted,  according  to  the  market,  to  get 
the  commercial  value ;  however,  assays  are  still  some- 
times reported  on  the  basis  of  $1.2929  per  Troy  ounce 
for  silver. 

There  are  two  simple  methods  of  calculating  the 
number  of  Troy  ounces  of  each  metal  per  ton  of  ore. 

^First — Suppose  20  grams  of  ore  are  taken  for  assay, 
and  2.42  mgs.  of  gold  are  obtained  from  this  amount 
of  ore ;  by  simple  calculation  we  have 

2l;fo-oXioo=.oi2i  per  cent. 

.0121  per  cent  of  29,166  (see  below)  =3.53  Troy  ounces 
of  gold  per  ton  of  such  ore. 

Second : — The  simpler  and  better  method  is  to  use 
the  assay  ton  (written  A.  T.)  system.  It  is  derived  as 
follows : 


I02  ASSAYING    GOLD 

One  avoirdupois  pound=7ooo  Troy  grains. 

2000  pounds=i  ton. 

2000X7000=14,000,000  Troy  grains  in  one  avoirdu- 
pois ton. 

480  Troy  grams=i  Troy  ounce. 

14,000,000-^480=29, 1 66-|-Troy  ounces  in  i  avoirdu- 
pois ton. 

The  unit  of  the  system  is  the  assay  ton=29.i66  grams, 
or  29,166  milHgrams.  Hence  milHgrams  correspond  to 
ounces,  and  there  will  be  as  many  ounces  of  metal  per 
ton  as  there  are  milligrams  of  metal  per  A.  T. 

Note. — In  assays  made  for  the  purpose  of  investigation  of  a 
process,  or  in  other  important  cases,  especially  with  rich  ore,  it  is 
advisable,  after  detaching  the  button,  to  return  the  slag  to  the 
pot,  remelt  it,  throw  in  some  litharge  mixed  with  carbon,  and, 
when  all  is  well  melted,  pour,  and  cupel  the  button.  The  result- 
ing bead  is  to  be  added  to  that  first  got  for  the  weighing.  Of 
course,  results  must  be  corrected  for  the  silver  contained  in  all 
of  the  litharge  which  has  been  employed. 

The  cupellation  loss  should  also  be  determined  and  corrected 
for,  either  by  the  method  shown  on  page  88,  or  by  assaying 
the  saturated  part  of  the  cupel,  as  directed  on  page  106,  which, 
however,  will  not  show  the  loss  by  volatilization. 


AND   SILVER   ORES  IO3 


ASSAY  OF  ORE  CONTAINING  COARSE  METAL. 


When  ore  contains  coarse  metal  which  can  not  be 
passed  through  the  sieve,  the  entire  sample  must  be 
weighed.  The  coarse  metal  which  remains  on  the  sieve 
must  also  be  weighed;  its  weight  deducted  from  that  of 
the  entire  sample  leaves  the  weight  of  the  powder. 
These  weights  must  be  noted;  the  sample  then  consists 
of  2  parts,  each  of  which  must  be  assayed.  If  the  coarse 
metal  is  not  in  too  large  quantity,  it  may  all  be  scorified 
with  its  weight  of  lead  and  a  little  borax,  and  the  button 
cupelled;  then,  as  the  weight  of  coarse  metal  is  to  the 
weight  of  fine  metal  in  the  bead,  so  is  the  weight  of  an 
ordinary  assay  to  the  weight  of  fine  metal  which  that 
quantity  of  coarse  metal  would  give;  from  this  the 
value  of  the  coarse  metal  per  ton  is  deduced.  The 
powder  having  been  assayed,  also,  we  find  that  the 
whole  sample  contained  so  many  grams  of  ore,  worth  so 
much  per  ton,  and  so  many  (the  coarse  metal),  worth 
so  much  per  ton;  this  is  just  the  same  as  if  we  had  so 
many  tons  of  each  quality,  instead  of  grams,  etc.,  and  all 
we  have  to  do  is  to  find  the  average  value  per  ton  of 
such  a  mixture.  To  do  this,  we  multiply  each  quantity 
by  its  value  per  ton,  and  add  the  results  together;  this 
gives  us  the  total  value  of  the  two  lots,  which,  divided 
by  the  united  weights,  gives  the  value  per  ton  of  the 
mixture. 

Suppose  the  total  weight  of  the  sample  to  be  565 
grams,   consisting  of   560   grams   of  powder,    assaying 


104  ASSAYING    GOLD 

$40   per  ton,   and   5   grams   of   coarse   metal,   assaying 
$30,420  per  ton,  then — 

560  tons  at  $40  per  ton  contains  $  22,400  00 
and       5     "  30,420    "  "  152,100  00 


and  565     "  "        $174,500  00 

Dividing  the  total  amount  by  the  total  number  of 
tons,  we  get  $308.85  as  the  value  of  one  ton  of  ore;  this 
is  a  simple  example;  if  the  ore  contained  both  gold  and 
silver,  it  would  be  necessary  to  calculate  for  each,  in 
order  to  get  the  value  of  the  ore  for  each. 

In  the  case  of  silver  ore  containing  native  silver  or 
silver  glance,  etc.,  the  method  generally  used  is  as  fol- 
lows :  The  entire  sample,  or  a  suitable  quantity  thereof, 
having  been  weighed,  is  finely  powdered  and  sifted.  The 
matter  remaining  on  the  sieve  is  not  weighed,  but  is 
either  scorified  with  lead  or  melted  in  a  crucible  with 
litharge,  borax,  and  soda,  adding  a  little  flour;  the  re- 
sulting button  is  cupelled,  and  the  value  of  the  fine  silver 
got  is  computed  for  the  entire  sample  at  so  much  per 
ton;  the  powder  is  also  assayed  in  the  usual  manner  for 
value  per  ton,  and  the  two  values  are  added.  The 
quantity  of  ore  to  be  taken  for  the  sample  depends  on  the 
coarseness  of  the  grains  of  metal;  it  should  be  sufficient 
to  give  a  good  average  of  the  lot.  If  the  sample  sub- 
mitted is  a  piece  of  moderate  size,  the  whole  must  be 
weighed  and  ground ;  in  other  cases,  as  a  sample  of 
ground  ore  or  concentrations,  a  convenient  quantity  is 
600  grams. 

Note. — In  case  a  few  particles  of  silver,  silver  glance,  etc., 
should  refuse  the  sieve,  if  the  proportion  of  these  to  the  entire 


AND   SILVER   ORES  IO5 

sample  is  small,  the  difficulty  may  be  overcome  by  dissolving  the 
tough  particles  in  nitric  acid,  adding  enough  hydrochloric  acid  to 
convert  all  of  the  silver  to  chloride,  drying  the  vv^hole,  and  grind- 
ing the  dried  mass  in  the  wedgewood  mortar  with  a  little  of  the 
ore  powder,  then  thoroughly  mixing  it  with  the  rest  of  the 
sample.  This  method  is  to  be  recommended  when  such  tough 
matter  is  encountered  at  a  rather  early  stage  in  the  grinding  and 
quartering  of  a  large  sample,  as  it  would  be  inconvenient  to  pass 
so  much  ore  through  the  finest  sieve. 

Sometimes  the  tough  matter  may  be  made  to  pass  by  repeatedly 
grinding  it  with  a  little  of  the  powder  which  has  already  passed 
through  the  sieve. 

If  the  metal  is  gold,  it  may  be  dissolved  in  aqua  regia,  the 
liquid  evaporated,  and  the  residue  ground  with  some  of  the 
powder  and  then  mixed  with  the  whole.  The  mixing  in  these 
cases  must  be  done  with  especial  care. 

When  a  small  mill  is  used  instead  of  the  pestle  and  mortar  for 
grinding,  it  sometimes  causes  the  coarse  metal  to  pass  the  sieve 
if  the  gangue  is  quartzose. 


I06  ASSAYING  GOLD 


ASSAY  OF  ROASTED  ORE  FOR  SOLUBILITY. 


This  is  to  ascertain  what  percentage  of  the  silver  in 
roasted  ore  can  be  extracted  by  leaching,  which  is  gen- 
erally equal  to  that  which  can  be  got  by  amalgamation. 
From  a  true  sample  of  the  roasted  ore,  weigh  out  two 
assays.  Leach  one  on  a  filter  of  coarse  filter-paper  with 
a  cold  solution  of  sodium  or  calcium  hyposulphite*  of 
density  about  5°  Beaume,  until  the  leach  gives  no  dark 
precipitate  on  addition  of  a  drop  of  solution  of  a  poly- 
sulphide.f  Dry  the  assay  (filter  and  all)  on  a  roasting- 
dish  in  the  muffle,  raising  the  heat  so  as  to  burn  the  filter. 

Dress  and  melt  the  leached  and  the  unleached  sam- 
ple. Cupel;  part  the  beads,  and  calculate  the  percent- 
age of  the  silver  extracted  from  the  leached  sample. 

To  make  a  polysulphide,  boil  quicklime  or  concen- 
trated lye  with  ground  sulphur  as  long  as  sulphur  is 
dissolved. 

To  Assay  a  Cupel. — When  it  is  desired  to  know 
how  much  silver  has  been  absorbed  by  a  cupel  during 
cupellation,  grind  the  saturated  part  of  the  cupel  very 
fine,  and  scorify  it  with  borax  glass  and  a  little  granu- 
lated lead,  or  make  a  crucible  fusion,  using  25  grams  of 
borax  glass,  25  grams  of  soda  and  i  gram  of  flour.  Cupel 
the  button. 

*Formerly  so  called;  now  called  thiosulphate,  the  name  hypo- 
sulphite being  applied  to  a  different  salt. 

tWith  ores  which  contain  lead,  the  work  is  much  hastened 
by  an  addition  of  sodium  carbonate  to  the  leaching  solution,  which 
prevents  the  lead  dissolving. 


AND   SILVER   ORES  I07 


ASSAY    BY    AMALGAMATION, 


Take  any  convenient  quantity  of  the  ore  powder, 
add  water  enough  to  make  a  pulp,  and  heat  it.  If 
gold  ore,  add  about  i-40th  per  cent  of  cyanide  of  po- 
tassium. If  silver  ore  (not  roasted),  add  2  per  cent  of 
salt,  and  such  other  chemicals  as  may  be  desired,  in  such 
proportion  as  can  be  profitably  used  on  the  large  scale. 
Agitate  with  pure  quicksilver  for  several  hours,  in  an  iron 
mortar  or  a  Buck's  amalgamator  with  the  muller 
slightly  raised  by  a  disc  of  iron  placed  on  the  top  of  the 
cone.  Separate  the  pulp  from  the  quicksilver  by  washing 
(panning).  If  gold  ore,  distil  the  quicksilver  or  dissolve 
it  in  dilute  nitric  acid  ;'the  gold  will  remain.  It  should  be 
melted,  either  before  the  blowpipe  or  otherwise,  to  drive 
of¥  any  remaining  quicksilver,  which  would  cause  spirting 
on  attempting  cupellation.  If  silver  ore,  strain  the 
quicksilver  through  a  piece  of  wet  buckskin.  (Twist  the 
buckskin  so  as  to  press  the  quicksilver  through.)  Tie 
the  amalgam  in  a  bit  of  cotton  cloth,  put  it  in  a  dry-cup, 
and  heat  slowly  to  redness  in  the  muffle.  Wrap  the 
metal,  gold  or  silver,  in  sheet  lead,  cupel,  part,  weigh 
and  calculate  results  by  percentage. 


I08  •  ASSAYING    GOLD 


ROASTED   SILVER   ORE  REQUIRES   NO 
CHEMICALS. 


The  results  obtained  from  gold  rock  in  this  way  are 
too  low,  because  some  of  the  gold  is  dissolved  in  the 
parting  from  quicksilver  by  nitric  acid.  It  is  better  to 
proceed  as  follows  : — 

After  amalgamating,  pan  out  very  carefully  into  a 
dish  or  pan  of  water.  Let  the  water  settle,  pour  it  into 
another  vessel,  leaving  the  ore  behind.  To  the  ore  add 
some  fresh  quicksilver,  and  agitate  for  some  time,  then 
pour  out  again  in  the  same  water.  Again  settle,  and 
pour  off  the  clear  water ;  dry  and  assay  the  tailings,  also 
assay  some  of  the  original  pulp.  The  difference  between 
the  two  assays  will  show  how  much  of  the  gold  has  been 
extracted. 

A  better  result  may  be  obtained  by  dissolving  the 
cyanide  of  potassium  in  hot  water,  and  dissolving  in 
that  solution  about  half  as  much  red  oxide  of  mercury, 
then  adding  it  to  the  pulp,  which,  if  acid  to  test-paper, 
should  first  be  neutralized  by  means  of  lime  or  soda. 
The  mercury  should  have  a  little  zinc  dissolved  in  it 
by  the  aid  of  heat. 


AND   SILVER   ORES  lOQ 


TO  FIND  THE  VALUE  OF  A  SPECIMEN. 


Handsome  specimens  of  gold  in  quartz  are  in  great 
demand.  There  are  many  rules  for  ascertaining  the 
proportion  of  the  gold  in  such  specimens,  all  based  on 
the  different  densities  of  the  specimens  as  a  whole,  of 
the  gold  and  of  the  quartz.  The  specific  gravity  of 
tl:e  gold  and  of  the  quartz  is  generally  assumed,  the 
former  at  from  17  to  19,  the  latter  at  2.6;  that  of  the 
specimen  is  found  by  weighing  it  in  air  and  in  water, 
and  dividing  its  weight  in  air  by  the  difference;  in  this 
case,  the  easiest  rule  to  remember  is  this : — 

Divide  the  specific  gravity  of  the  gold  by  that  of  the 
quartz  and  by  that  of  the  specimen.  From  the  greater 
quotient  subtract  the  lesser;  the  remainder  is  the  pro- 
l  ortion  of  gold.  From  the  lesser  quotient  subtract  i ; 
the  remainder  is  the  proportion  of  quartz. 

Then,  as  the  sum  of  these  proportional  quantities  is 
to  the  proportion  of  gold,  so  is  the  weight  of  the  speci- 
men to  the  actual  quantity  of  gold  in  it.  Suppose  a 
specimen  weighs  32  ounces  in  air  and  28  ounces  in 
water;  the  difference  is  4,  and  32  divided  by  4  gives  8, 
which  is  the  specific  gravity  of  the  specimen.  If  the 
specific  gravity  of  the  gold  is  assumed  to  be  17,  and 
that  of  the  quartz  2.6,  we  have  17  divided  by  2.6  gives 
6.539,  nearly,  and  17  divided  by  8  gives  2.125.  Sub- 
tracting the  lesser  quotient  from  the  greater  leaves 
4.414,  and  subtracting  i  from  the  lesser  leaves  1.125. 
and  the  proportion  is  4.414  of  gold  to  1.125  of  quartz. 


no  ASSAYING    GOLD 

Adding  these  together,  we  have  5.539.  Now  it  is  a 
mere  question  in  the  rule  of  three.  If  5.539  ounces  of 
the  specimen  contain  4.414  ounces  of  gold,  how  much 
does  the  whole  specimen,  weighing  32  ounces,  contain? 
Answer. — 25.5  ounces.  This  is  correct,  provided  the 
assumed  specific  gravities  of  the  gold  and  gangue  are 
correct,  but  if  greater  accuracy  be  desired,  proceed 
thus : — 

Weigh  the  specimen,  a  piece  of  the  metal,  and  a  piece 
of  the  gangue,  each  in  air  and  in  water.  Divide  the 
difference  between  the  respective  weights  in  air  and  in 
water  by  the  weights  in  air.  From  the  greater  quotient 
subtract  the  next  less,  and  from  that  the  least.  The 
first  remainder  is  the  proportional  weight  of  the  metal., 
the  second  is  that  of  the  gangue ;  the  sum  of  these  is 
that  of  the  specimen.  Having  the  actual  weight  of  the 
specimen,  that  of  the  metal  is  easily  found,  as  before, 
by  the  rule  of  three. 

Suppose  a  specimen  of  native  silver  in  spar  weighs 
84  pounds  in  air  and  73.5  in  water.  A  piece  of  the 
silver  weighs  27  grains  in  air  and  24  in  water.  A  piece 
of  the  spar  20  in  air  and  12  in  water,  then — 

Spar  in  air 20 

Spar  in  water   ....  12 

Difference   8^20=0.400 

Specimen  in  air  ...  84 
Specimen  in  water  73.5 

Difference    ....10.5-^-84=0.125 


AND   SILVER   ORES  III 

Silver  in  air 27 

Silver  in  water  ....  24 

Difference 3^27=0.111 

And  400 — i25=275=proportion   of   silver 
"      125 — 111=   14=         "  spar 

289^         "  specimen 

Then  289 1275 :  184 :79.93  pounds  of  silver  in  speci- 
men. 

The  difference  between  the  weight  in  air  and  in 
water  of  a  piece  of  rock,  metal,  etc.,  may  be  found  in 
several  ways,  according  to  the  facilities  at  hand,  as 
follows : — 

Weigh  the  object  on  scales  or  steelyard  of  suitable 
delicacy.  Suspend  the  object  by  means  of  a  hair,  a 
fine  thread,  or  a  wire,  according  to  its  weight,  under 
the  pan  of  the  scales  or  from  the  hook  of  the  steelyard ; 
submerge  it  in  water,  and  again  weigh  it.  Subtract  the 
second  weight  from  the  first.* 

Or,  weigh  the  object;  take  a  suitably-sized  vessel  full 
of  water,  or  in  which  the  water  stands  at  a  certain 
mark,  and  weigh  that  and  the  object  together,  if  con- 
venient, or,  if  not,  weigh  them  separately,  and  add  the 
results.  Now  remove  some  of  the  water,  place  the 
object  in  the  vessel,  replace  so  much  of  the  water  as  to 

*Some  scales  are  provided  with  a  hook,  from  which  the  object 
can  be  suspended  above  the  pan.  A  Httle  bench  of  sheet  brass  or 
tin  is  then  placed  across  the  pan,  leaving  the  pan  free  to  move 
beneath  it,  and  a  vessel  containing  water,  in  which  the  object  is 
submerged,  is  placed  on  the  bench. 


112  ASSAYING    GOLD 

raise  it  again  to  the  mark,  and  weigh  again.  The 
difference  between  the  last  weight  and  the  sum  of  the 
weights  of  the  object  and  of  the  vessel  of  water  is  the 
weight  of  the  water  displaced  by  the  object,  which  is 
equal  to  the  difference  of  the  weight  in  air  and  in 
water.  This  method  is  suitable  for  sand.  Special  bot- 
tles, called  specific-gravity  bottles,  graduated  to  hold 
a  certain  weight  of  water,  may  be  bought  with  counter- 
poise. Price,  $1.75  to  $3.50;  capacity,  100  to  1,000 
grains;  also,  100  grams. 

The  weight  of  the  object  in  air  divided  by  its  differ- 
ence in  water  is  the  specific  gravity.  The  difference 
divided  by  the  weight  in  air  is  the  specific  displace- 
ment. From  either  of  these  the  proportional  quanti- 
ties of  two  different  substances  composing  a  mixture 
can  be  determined  by  the  rules  given  for  the  valuation 
of  specimens. 

For  the  purpose  of  this  determination,  when  all  of 
the  substances  are  weighed  in  air  and  in  water,  it  is 
not  necessary  that  the  water  be  pure  or  at  standard 
temperature,  as  only  relative  displacements  or  specific 
gravities  are  required. 

Note. — Let  W.  be  weight  of  specimen  in  air,  and  let  D.  be  dif- 
ference of  weight  of  specimen  in  air  and  water,  then, 

(W.  X  s.  g.  metal) — (s.  g.  metal  X  s.  g.  gangue  X  D-) 

s.  g.  metal — s.  g.  gangue. 

=  weight  of  gold,  and 

(s.  g.  metal  X  s.  g.  gangue  X  D.)— (s.  g.  gangue  X  W.) 

s.  g.  metal — s.  g.  gangue 

:=  weight  of  gangue. 

The  determination  of  the  gangue  is  useful  as  a  check,  and  it  will 

be  observed  that  the  subtrahend  in  the  first  equation  is  the  minu- 


AND   SILVER   ORES  II3 

end  in  the  second,  while  the  divisor  is  the  same  in  each.  More- 
over, constants  may  be  kept  at  hand  for  assumed  s.  g.  of  gold  and 
gangue;  then,  at  17  and  2.6  respectively,  s.  g.  gold  X  s.  g. 
gangue  =44.2,  and  s.  g.  gold  —  s.  g.  gangue  =  14.4,  so  that  the 
work  in  the  first  example  given  is  only 

17X32=544 
44.2X4=176.8 

14.4)367.2(25.5  oz.  gold 

176.8 
2.6X32=832 

14.4)93.6(6.5  oz.  gangue 

**** 

The  following  method  is  especially  adapted  to  cases  in  which 
the  s.  g.  of  the  metal  and  of  the  gangue  are  neither  known  nor 
assumed : — 

Find  the  specific  displacement,  s.  d.,  of  the  metal  and  of  the 
gangue;  also  find  D.,  as  above,  then 

(W.Xs.  d.  gangue) — D. 

=  weight  of  gold. 

s.  d.  gangue  —  s.  d.  metal 
And 
D.  —  (W.  X  s.  d.  metal) 

■  =  weight  of  gangue. 

s.  d.  gangue— s.  d.  metal 

Taking  our  example  of  silver  in  spar: — 
84  X  .4  =  33.6 
10.5 

.4  —  .111  =  .289)  23.1  (  79.93  lbs.  metal 


114  ASSAYING    GOLD 

And 

10.5 
84  X  .111  =  9.324 

.288)1.176(4.07  lbs.  gangue 

As  in  the  preceding  example,  the  subtrahend  of  the  one  is  the 
minuend  of  the  other  equation,  and  the  divisor  is  the  same  for 
each;  also,  constants  may  be  got  for  given  s.  d.  of  metal  and 
gangue.  In  the  preceding  rule  it  is  not  necessary  to  calculate  s.  g. 
of  specimen ;  in  this  it  is  not  necessary  to  obtain  s.  d.  of  speci- 
men. Displacement  of  small  object  can  be  got  by  means  of  a 
Mohr  burette  very  easily. 


AND   SILVER   ORES  II5 


TESTS  FOR  ORES,  ETC. 


Silver,  Ag. — Powder,  mix  about  50  mgs.  with  twice 
as  much  fine  lead  (free  from  silver),  soda,  and  borax. 
Moisten  with  water,  place  in  cavity  scooped  in  piece  of 
charcoal,  smelt  by  blowpipe  flame,  cupel  the  lead,  exam- 
ine bead  with  lens,  if  large  enough  to  handle,  for  gold. 
Or,  powder,  roast,  boil  in  glass  or  earthen  vessel  with 
a  clean  strip  of  copper,  bluestone,  salt,  and  water;  gives 
a  white  coat  on  the  copper.  (Nearly  all  silver  ores  will 
do  so  if  boiled  with  bluestone  and  salt,  without  roast- 
ing.) 

Gold,  Au. — Powder,  roast  if  sulphurets  are  present, 
grind  very  fine,  and  wash  in  pan  or  horn;  examine 
with  lens ;  yellow  particles  not  soluble  in  nitric  acid. 

Copper,  Cu. — Powder,  moisten  with  salt  brine,  and 
throw  into  fire;  an  intensely  blue  flame.  Or,  moisten 
with  muriatic  acid,  and  direct  blowpipe  flame  on  it; 
the  same.  Or,  roast,  steep  in  ammonia;  a  blue  solu- 
tion. Or,  boil  in  nitric  acid  and  add  ammonia;  the 
same.* 

Lead,  Pb. — Powder,  mix  about  50  mgs  with  200  mgs. 
of  soda,  a  little  borax,  and  water  to  moisten;  place 
on  charcoal,  and  melt  under  blowpipe  flame;  gives  a 
malleable,  metallic  globule,  which,  when  melted  on  a 
cupel,  gradually  disappears,  leaving  a  yellow  stain  and. 
perhaps,   a  bead  of  silver,   or  a  coat  on  the    charcoal 

*Nickel  also  gives  a  blue  solution,  nearly  like  that  of  copper, 
with  ammonia,  but  it  does  not  give  the  blue  flame. 


Il6  ASSAYING    GOLD 

which  is  dark  lemon-yellow  hot,  sulphur-yellow  cold; 
disappears  on  being  heated,  but  not  touched,  by  the 
blowpipe  flame. 

Bismuth^  BI. — Powder,  treat  on  charcoal  same  as 
for  lead;  coat  darker  than  that  of  lead;  coat  heated 
but  not  touched  by  flame,  melts  into  brown  globule; 
globule  of  metal,  if  got,  is  brittle  and  reddish  in  color. 
It  becomes  covered  by  brown  crust,  which  cracks  during 
solidification,  showing  the  bright  metal,  or  the  globule 
gives  birth  to  a  smaller  one,  which  sprouts  from  its 
side  and  remains  bright;  best  seen  on  a  cupel.  This 
Is  the  only  metal  besides  lead  which  can  be  cupelled 
alone. 

Zinc,  Zn. — Powder,  throw  into  fire;  brilliant  white 
flame;  treated  On  charcoal  same  as  for  lead,  coat  yellow., 
hot ;  white,  cold ;  coat  heated,  but  not  touched,  by  flame, 
remains  and  is  luminous ;  touched  by  flame,  disappears ; 
moistened  by  a  drop  of  solution  of  cobalt  nitrate  and 
again  heated  to  glowing,  turns  deep  green  when  cold. 
To  detect  a  trace  of  zinc,  moisten  the  coal  with  a  solu- 
tion before  producing  the  coat.  Black  zinc-blende  is 
often  mistaken  for  galena.  The  two  may  be  distin- 
guished by  this  infallible  sign:  The  powder  of  galena 
Is  black,  that  of  blende,  brown  or  yellow. 

Tin,  Sn. — Powder,  mix  with  soda  (and  cyanide  of 
potassium  if  at  hand)  heat  strongly  on  coal,  covering 
with  blowpipe  flame;  white  metallic  globules,  which 
do  not  sink  into  a  cupel  when  melted  on  it,  but  form  a 
crust  which  is  yellow  hot.  Nitric  acid  converts  the 
globule  into  white  powder,  insoluble  in  water.  Coat 
on  coal  similar  to  zinc,  but  closer  to  bead ;  does  not  dis- 


AND   SILVER   ORES  II^ 

appear  when  heated,  whether  touched  by  flame  or  not. 
Treated  with  cobalt  solution,  is  bluish  green  when  cold. 
Rare  in  United  States. 

Antimony,  Sb. — Powder  treated  as  for  tin;  white 
coat  on  coal,  extending  a  long  way;  easily  driven  about 
by  flame;  globule,  if  obtained,  thrown  on  table  while 
hot;  rolls  along,  giving  off  dense  fumes.  Nitric  acid 
converts  the  globule  into  white  powder,  which  vanishes 
in  smoke  if  heated  on  charcoal 

Black  Oxide  of  Manganese,  MnOg. — Powder,  add 
muriatic  acid  or  sulphuric  acid  and  salt;  effervesces  and 
smells  of  chlorine,  which,  if  abundant  and  the  test  is 
made  in  a  test-tube  or  small  bottle,  may  be  seen  as 
yellowish-green  gas.  Melt  a  trace  with  borax  on  plati- 
num, beyond  the  point  of  the  flames ;  an  amethystine 
bead  indicates  manganese,  and,  as  it  evolves  chlorine 
from  muriatic  acid,  it  is  the  dioxide,  or  "black  oxide," 
and  an  oxidizer. 

Chromium  (in  chromates,  etc.),  Cr. — Powder,  melt 
a  very  little  with  borax  on  charcoal  in  blowpipe  flame; 
an  emerald  green  glass. 

Iron,  Fe. — Powder,  boil  in  muriatic  acid  and  a  drop 
of  nitric  acid;  add  a  drop  of  solution  of  potassium 
ferrocyanide  (yellow  prussiate  of  potash)  ;  a  deep  blue 
color  shows  iron  pr-esent.  But  treat  another  portion 
with  muriatic  acid  alone,  and  add  the  ferrocyanide;  if 
the  same  color  is  not  now  produced,  the  iron  is  not  in 
a  form  to  be  an  oxidizer. 

Tellurium,  Te. — Heat  a  fragment,  or  a  little  of  the 
powder   moistened,   by   blowpipe,   on   a   piece   of   white 


Il8  ASSAYING    GOLD 

porcelain;   moisten  the   still   hot  porcelain   with   strong 
sulphuric  acid;  a  red  color. 

Arsenic,  As. — Powder,  throw  on  live  coals,  or  heat 
on  charcoal,  within  the  blowpipe  flame ;  a  garlic  smell. 

Selenium,  Se. — Test  as  for  arsenic;  a  smell  of  rotten 
horseradish. 

Sulphur,  S. — Melt  with  soda  on  charcoal,  then 
moisten  and  place  on  a  piece  of  silver;  a  dark  stain 
indicates  sulphur."^  But  heat  to  redness  in  a  scorifier 
or  on  an  iron  spoon,  if  a  smell  like  that  of  burning 
matches  is  not  perceptible,  the  sulphur  is  not  in  a  form 
to  be  a  reducer. 

Quartz,  or  hard  silicate,  S'lO^. — Not  scratched  by 
knife,  not  affected  by  acid;  melted  with  2  parts  soda, 
froths  and  makes  clear  glass. 

Earthy  Carbonates  (lime,  CaO;  magnesia,  MgO; 
baryta,  BaO). — Scratched  by  a  knife,  foam  if  touched 
by  acid,  dissolve  with  effervescence  in  muriatic  acid  to 
a  colorless  solution;  burned  in  fire,  become  caustic 
quicklime,  and  no  longer  foam  with  acids. 

Earthy  Sulphates  (gypsum,  heavy  spar,  etc.). — 
Scratched  by  knife,  not  affected  by  acid;  show  sulphur 
by  the  test  with  silver,  but  give  no  smell  on  spoon. 

Nitrates  (Chile  niter,  KNO3,  etc.). — Flash  when 
heated  on  charcoal;  dissolved  in  water  with  sulphuric 
acid  and  salt,  can  dissolve  gold. 

Borates  (borax,  borate  of  Hme,  or  cotton  balls). — 
Moistened  with  glycerine  and  held  in  blowpipe  flame, 
give  green  color  to  it.     Or,  treat  with  sulphuric  acid 

*Unless  selenium  be  present,  which  also  gives  a  stain. 


AND   SILVER   ORES 


119 


in  porcelain  dish,  add  alcohol,  and  set  fire  to  it;  a  green 
flame.     No  copper  must  be  present  in  either  test. 

Alkaline  Carbonate  (of  soda,  NagCOg,  or  potash, 
K2CO3). — Dissolve  in  acids  with  effervescence,  and  do 
not  lose  this  property  by  being  melted;  dissolved  in 
water,  and  lime-water  added,  give  a  white  precipitate. 
Solution  turns  reddened  litmus-paper  blue. 

Clay. — Breathed  on,  gives  a  peculiar  smell,  easily 
recognized  when  once  known.  If  moistened,  becomes 
plastic.     If  dry,  sticks  to  the  tongue. 

Blowpipe. — Plain  brass  tube,  8  to  10  inches,  25  cents 
to  35  cents;  with  moisture-bulb,  8  to  10  inches,  50  cents; 
brass,  with  trumpet  mouth-piece,  condensing-chamber, 
and  platinum  jet,  $2.00;  same,  only  all  nickel  plated, 
$2.25 ;  Fletcher's,  with  hot  blast  (the  tube  encircles  the 
flame  of  the  lamp,  and  becomes  heated  and  dries  the 
air),  $1.00. 

By  placing  the  tip  within  the  flame  of  a  candle  or 
lamp,  and  blowing  into  the  larger  end,  a  pointed  flame 
is  produced.  To  make  a  cupel  for  use  with  the  blow- 
pipe, scoop  a  circular  cavity  in  a  piece  of  charcoal,  fill 
it  with  very  fine  moistened  bone-ashes,  press  with  a  large 
bullet  or  other  convex  object,  and  dry.  A  button  to 
be  cupelled  should  be  heated,  but  not  touched,  by  the 
flame. 

To  make  test-lead  free  from  silver. — Dissolve  acetate 
of  lead  in  water,  and  add  a  piece  of  zinc.  Wash  the 
resulting  lead  with  water  and  a  little  sudphuric  acid, 
then  with  water.  Dry  and  test  for  silver,  which  will 
not  be  found  unless  the  zinc  contained  it,  in  which 
case  the  zinc  is  not  fit. 


I20  ASSAYING    GOLD 


A  FEW  SPECIAL  MINERALS. 


Argentite,  or  Silver  Glance  {A g^S ^Silver  and 
Sulphur,  8y  per  cent  silver) . — Black,  heavy,  soft,  mallea- 
ble, sectile  (can  be  cut  in  slices).  Alone,  BB"*"  yields 
nearly  pure  silver. 

Cerargyrite,  or  Horn  Silver  (AgCl^=Silver  and 
Chlorine,  75  per  cent  silver). — Pearly,  yellowish,  or 
greemish;  exposed  to  light,  turns  dark;  heavy,  waxy, 
malleable,  sectile.  With  soda  BB  yields  pure  silver. 
Iodide  and  bromide  are  very  similar. 

Pyrargyrite,  or  Dark  Red  Silver  (Ag^^S^Sb^Sil- 
ver,  Sulphur,  and  Antimony). — Red  to  black,  powder 
red,  brittle,  tender.     BB  with  soda  yields  silver. 

Proustite,  or  Light  Red  Silver  (Ag^S^As^^Silver, 
Sulphur  and  Arsenic). — Light  red,  powder  red,  brittle, 
tender.*  BB  gives  garlic  odor,  and  with  soda  yields 
silver. 

Stephanite,  or  Brittle  Silver  (Ag^S^Sb^Silver, 
Sulphur  and  Antimony). — Black,  powder  black,  brittle, 
and  very  tender.     BB  with  soda  yields  silver. 

Galena  {PbS=Lead  and  Sulphur,  86  1-2  per  cent 
lead). — Nearly  black,  shining,  heavy,  rather  tender, 
usually,  but  not  always,  breaks  in  square  blocks.  The 
fine-grained  kind  generally  carries  antimony.  BB  with 
soda  yields  lead. 

*BB  means  before  blowpipe,  on  charcoal. 


AND   SILVER   ORES  121 

Cerussite,  or  Carbonate  of  Lead  (PbCO^^Lead 
Oxide  and  Carbonic  Acid,  66  per  cent  lead). — White, 
yellow,  or  nearly  black,  greasy  looking,  rather  hard, 
froths  with  acid.    Alone,  BB  yields  lead. 

Iron  Pyrites  (FeS^^^Sulphur  and  Iron). — Yellow  to 
bronze,  shining,  not  scratched  by  knife,  sometimes 
strikes  fire  with  steel.  *  Alone  BB  gives  abundant  fumes 
of  sulphur.  When  roasted,  before,  not  in,  the  flame  (or 
in  the  muflle)  until  it  smells  no  more,  is  a  red  powder. 
Arsenical  pyrites  contain  also  arsenic;  color,  silver 
white.    Often  carry  gold. 

Chalcopyrite,  or  Copper  Pyrites  {CuFeSo^^Copper, 
Iron  and  Sulphur). — Yellow  shining,  often  iridescent  on 
surface,  scratched  by  knife;  BB  with  soda  (after  roast- 
ing) gives  grains  of  copper  and  iron.  Purple  copper  is 
very  similar.     Sometimes  carry  silver. 

Chalcocite,  or  Copper  Glance  (Cu^S^Copper  and 
Sulphur). — Nearly  black,  rather  soft  sectile.  BB  with 
soda  (after  roasting),  yields  copper.  This  mineral  is 
sometimes  mistaken  for  silver  glance.  The  two  may 
occur  combined. 

Gray  Copper  {Copper,  Zinc,  Antimony,  and  Arsenic, 
and  sometimes  Silver  and  Mercury). — Dark  gray.  (See 
tests  for  silver,  etc.) 

Malachite,  or  Copper  Carbonates  (Cu^CO^^Cop- 
per  Oxide  and  Carbonic  Acid). — Bright  blue  or  green, 
often  crystallized,  froth  with  acid,  dissolve  in  ammonia 
with  beautiful  blue  color.    BB  with  soda  yield  copper. 

Cuprite,  or  Red  Copper  {Cu^O^^Copper  and  Oxy- 
gen).— Deep  red,  tender,  does  not  froth  with  acid,  dis- 


122  ASSAYING    GOLD 

solves    in    ammonia,    and    the    solution    turns    blue    by 
exposure.     BB  with  soda  yields  copper. 

Sphalerite,  or  Zinc-blende  (ZnS=:Zinc  and  Sul- 
phur).— Black,  red,  yellow,  green,  or  colorless  (rarely), 
powder  always  light  colored.  (See  test  for  zinc  and 
sulphur.) 

Stibnite,  or  Sulphuret  of  Antimony  {Sh^S^-^Anti- 
mony  and  Sulphur). — Gray,  shining,  usually  fibrous  or  in 
stars,  tender,  and  will  melt  in  candle-flame.  Alone,  BB 
flies  away  in  smoke,  leaving  white  coat  on  coal.  (See 
test.) 


AND   SILVER   ORES 


123 


SOLUBILITY    OF    METALS. 


The  following-named  metals  act  as  stated  in  the  dif- 
ferent liquids  with  the  aid  of  heat.  It  must  be  under- 
stood that  not  the  ores  of  metals,  but  the  metals  them- 
selves, as  melted  out  by  blowpipe  or  otherwise,  are 
meant : — 


In  moderately  strong  nitric  acid  :- 
Dissolve, 


Silver,  copper,  iron,  lea(^, 
bismuth,    zinc,    mercury. 


Not, 


Gold,  Platinum.  Tin  and 
antimony  are  converted 
into  white  powder. 


In  strong  sulphuric  acid : — 

Dissolve,  ! 

Silver,  copper,  bismuth, 
zinc,  tin,  mercury,  anti- 
mony. Lead  is  converted 
into  sulphate,  and  dis- 
solved if  enough  acid  is 
present. 

In  dilute  sulphuric  acid  :- 

Dissolve, 
Iron,  zinc,  tin. 


Not, 
Gold,      platinum,      iron. 
Lead  is  converted  into  sul- 
phate,    and     partly      dis- 
solved. 


Not, 
Gold,     platinum,    silver, 
copper,       lead,       bismuth, 
mercury. 


124 


ASSAYING    GOLD 


In  hydrochloric  acid: — 

Dissolve, 
Iron,   zinc,  bismuth,  an- 
timony   in    powder.      Tin 
slowly  in  an  abundance  of 
strong  acid. 

In  mixed  nitric  and  hydrochloric  acids 
Dissolve, 
Gold,  platinum,  copper, 
iron,  bismuth,  lead  (with 
water),  zinc.  Tin  and 
antimony  with  excess  of 
hydrochloric  acid. 

In  solution  of  caustic  potash,   zinc  and  tin   dissolve 
also  aluminum. 


Not, 
Gold,    platinum,     silver, 
copper     (unless     in     very 
fine    powder),    lead,    mer- 
cury. 


Silver  and  mercury  are 
acted  on,  but  not  dissolved. 
Antimony  remains  as  a 
white  powder  if  nitric  acid 
is  in  excess. 


AND   SILVER   ORES  1 25 


SUBSTITUTES    AND    EXPEDIENTS. 


If  nitric  acid  is  not  at  hand,  use  instead  a  solution 
of  niter  to  which  some  sulphuric  acid  is  added ;  this  must 
be  tested  for  chlorine,  as  directed  for  nitric  acid,  before 
being  used  for  parting.  For  hydrochloric  acid,  take  a 
solution  of  common  salt  and  add  sulphuric  acid  to  it. 

If  litharge  can  not  be  conveniently  got,  ground  galena 
may  be  used  in  its  place  in  crucible  assays  by  the  second 
system;  it  should  contain  no  gold  and  but  little  silver, 
and  the  assays  must  be  corrected  for  that  which  it  does 
contain;  the  weight  used  shouldbe  at  least  equal  to  that 
of  the  ore,  unless  the  latter  contains  lead,  when  less  will 
answer.  Lead  may  be  obtained  from  galena  by  fusion 
in  a  crucible  with  pieces  of  iron,  iron  ore,  or  soda;  it 
may  be  granulated  by  means  of  a  rasp. 

Ground  glass,  mixed  with  a .  little  soda,  will  answer 
in  place  of  borax. 

The  baking  soda  sold  in  the  stores  will  serve  as  well 
as  any  for  making  assays;  also  soda  ash. 

Niter  may  be  replaced  in  ore  assays  by  manganese 
black  oxide,  or  iron  red  oxide,  if  a  plenty  of  borax  or 
glass  be  used. 

Bone  ashes  may  be  made  by  burning  and  grinding 
bones;  the  soft  bone  from  the  horns  of  cattle  is  best. 

A  common  table  knife  will  answer  for  a  spatula. 

A  coarse  sieve  may  be  readily  made  from  a  tin  pan 
by  suitably  puncturing  its  bottom;  a  fine  one  by  stretch- 
ing a  piece  of  gauze  on  a  frame  or  hoop  of  wood  or 
metal. 


126  ^  ASSAYING    GOLD 

A  flat  rock  will  answer  for  a  mortar  or  grinding- 
plate,  a  large  pebble  or  small  boulder  for  a  pestle  or 
muller. 

A  crucible  laid  on  its  side  will  serve  as  a  muffle.  A 
quicksilver  flask,  of  which  one  end  has  been  removd, 
is  better;  a  hole  should  be  made  in  the  rear  end  of  the 
flask. 

A  furnace  may  be  built  of  rocks  and  mud,  or  of  mud 
alone,  or  adobes.  A  blacksmith's  forge  may  be  used 
for  both  melting  and  cupelling,  the  cupel  being  placed 
on  a  piece  of  brick  or  iron  in  the  midst  of  the  fire,  or 
in  any  kind  of  muffle. 

A  scorifier  or  a  roasting-dish  may  be  formed  by  hand 
from  good  white  clay,  or  carved  from  soap-stone. 

Cupels  may  be  made  by  pressing  moistened  bone 
ashes  into  suitably-sized  rings  of  tin,  sheet  or  hoop 
iron;  the  cupels  so  made  are  used  with  the  rings  still 
on  them. 

Pincettes  may  be  made  by  bending  a  strip  of  sheet 
metal  pointed  at  both  ends,  or,  for  coarser  ones,  a  piece 
of  stout  wire,  the  ends  flattened  by  filing. 

For  an  anvil,  any  flat,  smooth  piece  of  iron  will  do; 
the  head  of  an  ax  or  hatchet  struck  into  a  block. 

A  strip  of  paper  folded  lengthwise  forms  as  good  a 
test-tube  holder  as  need  be,  by  being  doubled,  the  tube 
in  the  loop  and  the  ends  held  in  the  hand. 

A  teacup  or  part  of  one  will  serve  for  parting  in, 
being  warmed  with  care.  A  Chinese  small  porcelain 
basin  is  better,  and  is  useful  in  other  ways. 

A  water-bath  is  any  vessel  containing  boiling  water 
on  which  a  pan  or  dish  can  be  placed  as  a  lid. 


AND   SILVER  ORES 


127 


A     SIMPLE    ASSAY     BALANCE. 


A  Balance  for  weighing  pulp,  buttons,  etc.,  and  one 
for  beads  may  be  made  as  shown  in  the  cut. 

The  beam  A  is  of  woods  the  support  B  and  pan  D  of 
tin ;  the  counterpoise  E  of  lead.  Two  needles  passed 
through  the   beam   form  the  points   of   suspension.     A 


-.    IHoSCIlAfrf^'CPfHiS 


second  and  lighter  rider  or  counterpoise  will  be  found 
useful  for  fine  weighing;  it  may  be  made  of  wire  or 
paper. 

The  center  of  gravity  of  the  beam  must  be  neither 
too  high  nor  too  low;  if  too  low,  the  instrument  will 
not  show  plainly  a  sufficiently  small  difference  of  weight ; 
if  too  high,  the  beam,  when  nearly  equally  loaded  on  the 
two  ends,  will  fall  to  either  side  and  there  remain;  it 
will  be  top-heavy,  and  will  not  swing;  it  must  be  ad- 
justed by  cutting  away  the  upper  or  lower  edge,  or  by 
loading  either  as  may  be  required. 

Two  such  scales  will  be  required,  one  of  about  the 


128  ASSAYING    GOLD 

size  shown  in  the  illustration,  for  weighing  beads,  and 
one  much  larger,  for  pulp,  fluxes,  buttons  of  lead  or 
copper,  etc. 

It  will  be  found  convenient  to  arrange  a  support 
under  the  end  E  of  the  beam,  in  order  to  limit  its  range 
of  movement.  To  weigh  a  bead  or  button,  put  it  on 
the  pan,  and  remove  the  riders  until  the  beam  is  exactly 
balanced,  then  move  the  bead,  and  put  weights  on  the 
pan  until  the  counterpoise  is  again  balanced;  the 
weights  required  are  equal  to  the  bead  or  button.  A 
well-made  small  scale  will  work  correctly  to  i-ioo  grain 
or  less. 

To  weigh  the  pulp  for  an  assay,  place  the  proper 
weight  on  the  pan,  and  adjust  the  counterpoise  to  bal- 
ance it,  then  remove  the  weight,  and  put  pulp  on  the 
pan  until  it  again  balances ;  the  weight  of  pulp  is  equil  to 
that  which  it  replaces.  Lift  the  pan  off  the  beam,  and 
^  our  the  pulp  into  the  crucible. 

A  cheap  lens  may  be  made  by  placing  a  drop  (  f 
vater  on  a  clean,  dry  piece  of  window-glass. 


AND   SILVER   ORES 


129 


ASSAY  TABLE. 


For  Two  Hundred  and  Forty  Grains  of  Ore. 


Bead 
Points. 

Ounces  IN  A  Ton 
OF  THE  Ore. 

Value   (in  dollars) 

OF  Silver. 

Value  (in  dollars) 
OF  Gold. 

1 

u.   t.  h.  th. 

1215278 

u.    t.  h.  th. 

1571261 

u.  t.  h.  th. 

25122067 

2 

2430556 

3142523 

50244134 

3 

3645834 

4713784 

75366202 

4 

4861 112 

6285045 

100488269 

5 

6076390 

7856307 

125610336 

6 

7291668 

9427568 

150732403 

7 

8506946 

10998830 

1 75854470 

8 

9722224 

12570091 

200976537 

9 

10937502 

14141352 

226098605 

Find  the  right-hand  figure  of  bead-weight  in  the  column 
of  "Bead  Points,"  and  the  figures  for  ounces  or  value 
under  the  corresponding  head.  Set  the  figures  down,  or 
as  many  of  them  as  may  be  wished,  pointing  off : — 

For  tenths    u.  etc. 

For  units all  to  the  right  of  u. 

For  tens    "  "  t. 

For  hundreds   *'  "  h. 

For  thousands    *'  '*         th. 


130  ASSAYING    GOLD 

• 

Proceed  then  with  the  next  figure,  setting  down  this 
value  with  the  decimal  point  under  that  of  the  first,  and 
so  on,  then  add  all  together.  Example:  A  bead  equals 
42.7,  required  the  ounces  in  a  ton : — 

•  7  tenths 85 

2-units 2.43 

4 — tens    48.61 


42.7  '  51-89  ounces. 

It  will  be  found  convenient  to  take  a  pointer  (a  dry  pen 
or  a  pencil),  and,  when  inspecting  the  table,  place  it  be- 
tween the  figures  where  the  decimal  point  will  fall,  and 
then  read  and  transcribe  the  value.  In  this  way  the 
superfluous  figures  to  the  right,  say,  beyond  two  or  three 
places  of  decimals,  need  not  be  noticed,  and  all  risk  of 
confusion  will  be  avoided.  A  little  practise  will  be  needed 
to  give  facility. 


AND   SILVER   ORES 


131 


ASSAY   TABLE. 


For  Twenty  Grams  of  Ore. 


Bead 
Points. 

Ounces  in  a  Ton  of 
THE  Ore. 

Value  in  Gold. 
(Dollars.) 

1 

u.  t.  h.  th. 

145833 

K.  t.  h.  th. 

3014642 

2 

291667 

6029284 

3 

4 

437500 
583333 

9043926 

K.  t.  h.  th. 

12059567 

5 

729167 

15073209 

6 

7 

875000 

u.  t.  h.  th. 

1020833 

18087851 
21 102493 

8 

1166667 

241 17135 

9 

1312500 

27131777 

This  table  is  used  in  the  same  manner  as  the  preceding 
one;  there  is,  also,  another  use  to  which  it  can  be  put, 
which  the  author  believes  to  have  been  first  noticed  by 
himself ;  it  is  finding  the  equivalent  in  troy  ounces  of  any 
number  of  pounds  and  decimals  of  a  pound  avoirdupois. 
Multiply  the  pounds  by  10,  consider  the  product  as  milli- 
grams of  metal  got  from  a  20-gram  assay,  and  find  by  the 
table  the  corresponding  number  of  ounces.  To  find  the 
value  of  a  bar  of  which  the  weight  is  given  in  pounds. 


132  ASSAYING    GOLD 

multiply  the  pounds  by  the  fineness  and  that  product  by 
10;  consider  the  last  product  as  milligrams  of  gold  or 
silver,  as  the  case  may  be,  and  take  the  values  from  the 
table  as  though  for  an  ore  assay. 


AND   SILVER   ORES 


133 


ASSAY   TABLE. 


For  One  Assay  Ton  of  Ore. 


Ounces. 

Gold— Value. 

Silver — Value. 

1 

u.    t.    h.  th. 

$  2067183 

u.  t.  h.  th. 

$  1292929 

2 

4132367 

2585858 

3 

6201550 

3878788 

4 

8278734 

5171717 

5 

10335917 

6464646 

6 

12403101 

7757576 

7 
8 

14470284 
16537468 

9050505 

«.  /.  h.ih. 

1 0343434 

9 

18604651 

1 1 636364 

Bancroft  LilifVf 

This  table  is  used  in  the  same  way  as  the  others,  but  as 
the  ounces  per  ton  are  got  at  once  from  the  bead-weight, 
only  values  per  ton  are  required;  it  is  also  applicable  to 
finding  the  value  of  bars,  and  the  value  per  ounce  of 
bullion  of  any  given  fineness. 

To  find  the  value  of  any  number  of  ounces  of  bullion 
•of  any  fineness,  multiply  the  weight  in  troy  ounces  by 
the  fineness ;  the  product  is  the  weight  of  the  pure  metal. 
Find  the  value  of  the  pure  metal  from  the  table. 


134 


ASSAYING    GOLD 


Example:    A  silver  bar  weighs   1,642  ounces,  and  is 
.840  fine,  what  is  its  value? 

1642X -840=  1379.28  ounces  of  pure  silver  in  the  bar : — 
Then,  from  the  table, 

$1,292.93 

387.88 

90.50 

11.63 

0.26 

0.01 


1,000      oz. 

z= 

300     '' 

= 

70    " 

= 

9      " 

= 

0.2    " 

=1: 

0.08  " 

3= 

1,379.28  oz.  r= $1,783.21 

One  ounce  of  bullion  one  thousand  fine  is  of  the  same 
value  as  one  thousand  ounces  one  fine;  or  372  ounces 
1,000  fine  =  1,000,  .372  fine. 

To  find  the  value  of  one  ounce  of  bullion  of  any  fine- 
ness : — 

Consider  the  "points"  of  fineness  as  so  many  ounces 
1,000  fine,  and  find  the  value  of  that  quantity  from  the 
table.  Divide  by  1,000  (move  the  decimal  point)  ;  the 
quotient  will  be  the  value  of  one  ounce  at  the  given 
fineness. 

Example :  What  is  the  value  per  ounce  of  silver  bul- 
lion .840  fine  ? 

800  oz.  1,000  fine  =:  $1,034.34 
40    "       "       "     =         51.72 


$1,086.06 


And  $1,086.06 -^  1,000  =  $1,086  per  oz. 

All  these  calculations  are  made  on  the  old  basis  of 


AND   SILVER   ORES  1 35 

$1.2929  per  ounce  for  silver;  the  values  found  must  be 
corrected  to  correspond  with  the  commercial  value.  This, 
however,  is  not  the  case  with  gold,  which  is  always  reck- 
oned at  $20.67  P6^  ounce  of  pure  metal,  and  the  value  of 
a  gold  bar  can  be  got  from  the  table  in  the  same  way  as 
that  of  the  silver  bar. 

The  following  tables  will  be  convenient  to  those  who 
may  wish  to  make  a  full  set  of  Assay  Ton  weights  to  go 
with  either  grain  or  gram  assay-ounce  weights.  The 
various  pieces  may  be  made  of  any  convenient  metal, 
as  lead,  brass,  or  silver.  Below  o.oi  A.  T.  they  are  best 
made  of  aluminum,  because  that  metal,  being  light,  allows 
them  to  be  of  good  size;  but  these  lower  fractions  will 
not  be  required  in  weighing  fluxes,  flour,  etc.,  so  that  they 
may  be  omitted,  unless  it  is  intended  to  weigh  the  prills 
of  metal  from  lead,  tin,  bismuth,  etc.,  assays  with  them. 
All  of  the  weights  above  o.oi  A.  T.  may  be  adjusted  by 
means  of  the  pulp-balance;  below  that,  the  assay  scales 
should  be  used.  To  make  a  weight,  cut  a  piece  of  metal 
so  that  it  is  a  little  too  heavy,  and  then  trim  it  carefully  by 
cHpping  and  filing  unil  it  is  of  the  right  weight.  Begin 
with  the  larger  ones,  then,  if,  by  accident,  you  make  a 
piece  too  light,  it  may  be  used  for  the  next  smaller.  By 
combining  the  pieces,  any  fraction  of  an  A.  T.  can  be 
weighed  to  i-ioo,  corresponding  to  20  pounds  in  the  real 
ton,  or  with  the  full  set,  to  i-iooo,  corresponding  to  2 
pounds. 


136 


ASSAYING    GOLD 


GRAIN   SYSTEM. 


One  One-Hundredth  Grain  Is  One  Assay  Ounce. 


1 

0.5 

0.2 

0.2 

0.1 

0.05 
0.02 
0.02 
0.01 

0.005 
0.002 
0.002 
0.001 


A.  T.  Contains 291.667  Grams 

145.833 
58.333 

58.333 

29.167 

14.583 

5.833 

5.833 

2.917 

1.458 

0.583 

0.583 
0.292 


AND   SILVER   ORES 


137 


GRAM     SYSTEM. 


One   Milligram   Is   One  Assay   Ounce. 


1       A.T. 

CONTi 

0.5 

0.2 

0.2 

0.1 

0.05 

0.02 

0.02 

0.01 

0.005  ' 

0.002   ' 

0.002  ' 

0.001   ' 

Contains 29.1667  Grams 

14.5833 

5.8333 

5.8333 

2.9167 

1.4583 

0.5833 

0.5833 
0.2917 

0.1458 

0.0583 

0.0583 

0.0292 


In  working  with  the  gram  system,  the  operator  must 
not  omit  to  observe  whether  his  assay-weights,  with 
which  the  beads  are  to  be  weighed,  are  i  gram=:  1,000. 
in  which  case  the  unit  is  a  milligram,  or  an  Assay  Ounce 
with  reference  to  the  A.  T.,  or  ^  gram  =  1,000,  when  the 
unit  is  a  half  milligram,  or  a  half  Assay  Ounce  with  ref- 
erence to  the  A.  T.,  and  therefore  an  Assay  Ounce  to  the 
half  A.  T. 

In  the  first  case,  the  bead-weight  as  obtained  from  the 


138  ASSAYING    GOLD 

assay-balance  is  the  number  of  ounces  in  a  ton  if  an  A.  T. 
of  ore  was  used ;  in  the  second  case,  the  bead-weight  is 
the  number  of  half  ounces  in  a  ton  if  an  A.  T.  of  ore 
was  taken,  hence  must  be  divided  by  2  for  the  ounces,  but 
it  is  the  number  of  ounces  to  the  ton  if  a  half  A.T.  of  ore 
was  used. 


AND   SILVER  ORES  139 

TABLE    OF    VALUES    OF    GOLD 

OF  GOLD  IS 
WEIGHT 

WORTH 

I  grain  Troy  $0.0430 

I  gram=i5.43    grains   Troy 0.6646 

I  pennyweight  Troy=:24  grains  Troy I-0335 

I  ounce  Troy=20  pennyweights  Troy=48o 

grains  Troy=i3i.io  grams 20.6718 

I  ounce  Avoirdupois=437^  grains  Troy= 

28.35    gi'ams 18.8415 

I  pound  Troy^  1 2  ounces  Troy=240  penny- 
weights    Troy  =5,760      grains      Troy= 

373.24    grams 248.0620 

I  pound  Avoirdupois=i6  ounces  Avoirdu- 

pois=:7,ooo  grains  Troy=453.59  grams..       301.4642 
I  ton  Avoir.=2,ooo  pounds  Avoir.  =  (29. 166 
ounces    Troy;    32,000    ounces    Avoir. )==: 
14,000,000  grains   Troy 602,928.4660 


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THE  .  .  . 

METALLURGY  OF  COMMON  METALS 

(GOLD.  SILVER,  IRON,  COPPER.  LEAD,  AND  ZINC) 

BY 

Leonard  S.  Austin 

Professor  of  Metallurgy  and  Ore  Dressing,  Michigan  College  of  Mines,  Houghton,  Mich. 


First  Edition,  1906 


This  text-book  incorporates  the  20  years'  experience  of 
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chapters  upon  cyaniding  have  been  carefully  corrected  by 
Mr.  Francis  L.  Bosqui,  the  well-known  cyanide  expert. 

Besides  a  full  treatment  of  the  metallurgy  of  the  metals 
named,  several  chapters  are  devoted  to  the  preparation  of 
ores  by  sampling,  crushing,  and  roasting;  to  fuel  and  refrac- 
tories;  to  thermo-chemistry,  and  to  the  refining  of  metals. 

Much  attention  is  given  to  the  location  and  construction 
of  metallurgical  plants,  to  their  organization  and  manage- 
ment, and  to  the  commercial  considerations  involved  in  the 
handling  of  ores  and  metals. 


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